test: add comprehensive delivery engine and circuit breaker tests

Add unit tests for internal/delivery/ package covering:

Circuit breaker tests (circuit_breaker_test.go):
- Closed state allows deliveries
- Failure counting below threshold
- Open transition after threshold failures
- Cooldown blocks during cooldown period
- Half-open transition after cooldown expires
- Probe success closes circuit
- Probe failure reopens circuit
- Success resets failure counter
- Concurrent access safety (race-safe)
- CooldownRemaining for all states
- CircuitState String() output

Engine tests (engine_test.go):
- Non-blocking Notify when channel is full
- HTTP target success and failure delivery
- Database target immediate success
- Log target immediate success
- Retry target success with circuit breaker
- Max retries exhausted marks delivery failed
- Retry scheduling on failure
- Exponential backoff duration verification
- Backoff cap at shift 30
- Body pointer semantics (inline <16KB, nil >=16KB)
- Worker pool bounded concurrency
- Circuit breaker blocks delivery attempts
- Circuit breaker per-target creation
- HTTP config parsing (valid, empty, missing URL)
- scheduleRetry sends to retry channel
- scheduleRetry drops when channel full
- Header forwarding (forwardable vs hop-by-hop)
- processDelivery routing to correct handler
- Truncate helper function

All tests use real SQLite databases and httptest servers.
All tests pass with -race flag.

internal/delivery/circuit_breaker_test.go

@@ -0,0 +1,243 @@
+package delivery
+
+import (
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCircuitBreaker_ClosedState_AllowsDeliveries(t *testing.T) {
+	t.Parallel()
+	cb := NewCircuitBreaker()
+
+	assert.Equal(t, CircuitClosed, cb.State())
+	assert.True(t, cb.Allow(), "closed circuit should allow deliveries")
+	// Multiple calls should all succeed
+	for i := 0; i < 10; i++ {
+		assert.True(t, cb.Allow())
+	}
+}
+
+func TestCircuitBreaker_FailureCounting(t *testing.T) {
+	t.Parallel()
+	cb := NewCircuitBreaker()
+
+	// Record failures below threshold — circuit should stay closed
+	for i := 0; i < defaultFailureThreshold-1; i++ {
+		cb.RecordFailure()
+		assert.Equal(t, CircuitClosed, cb.State(),
+			"circuit should remain closed after %d failures", i+1)
+		assert.True(t, cb.Allow(), "should still allow after %d failures", i+1)
+	}
+}
+
+func TestCircuitBreaker_OpenTransition(t *testing.T) {
+	t.Parallel()
+	cb := NewCircuitBreaker()
+
+	// Record exactly threshold failures
+	for i := 0; i < defaultFailureThreshold; i++ {
+		cb.RecordFailure()
+	}
+
+	assert.Equal(t, CircuitOpen, cb.State(), "circuit should be open after threshold failures")
+	assert.False(t, cb.Allow(), "open circuit should reject deliveries")
+}
+
+func TestCircuitBreaker_Cooldown_StaysOpen(t *testing.T) {
+	t.Parallel()
+	// Use a circuit with a known short cooldown for testing
+	cb := &CircuitBreaker{
+		state:     CircuitClosed,
+		threshold: defaultFailureThreshold,
+		cooldown:  200 * time.Millisecond,
+	}
+
+	// Trip the circuit open
+	for i := 0; i < defaultFailureThreshold; i++ {
+		cb.RecordFailure()
+	}
+	require.Equal(t, CircuitOpen, cb.State())
+
+	// During cooldown, Allow should return false
+	assert.False(t, cb.Allow(), "should be blocked during cooldown")
+
+	// CooldownRemaining should be positive
+	remaining := cb.CooldownRemaining()
+	assert.Greater(t, remaining, time.Duration(0), "cooldown should have remaining time")
+}
+
+func TestCircuitBreaker_HalfOpen_AfterCooldown(t *testing.T) {
+	t.Parallel()
+	cb := &CircuitBreaker{
+		state:     CircuitClosed,
+		threshold: defaultFailureThreshold,
+		cooldown:  50 * time.Millisecond,
+	}
+
+	// Trip the circuit open
+	for i := 0; i < defaultFailureThreshold; i++ {
+		cb.RecordFailure()
+	}
+	require.Equal(t, CircuitOpen, cb.State())
+
+	// Wait for cooldown to expire
+	time.Sleep(60 * time.Millisecond)
+
+	// CooldownRemaining should be zero after cooldown
+	assert.Equal(t, time.Duration(0), cb.CooldownRemaining())
+
+	// First Allow after cooldown should succeed (probe)
+	assert.True(t, cb.Allow(), "should allow one probe after cooldown")
+	assert.Equal(t, CircuitHalfOpen, cb.State(), "should be half-open after probe allowed")
+
+	// Second Allow should be rejected (only one probe at a time)
+	assert.False(t, cb.Allow(), "should reject additional probes while half-open")
+}
+
+func TestCircuitBreaker_ProbeSuccess_ClosesCircuit(t *testing.T) {
+	t.Parallel()
+	cb := &CircuitBreaker{
+		state:     CircuitClosed,
+		threshold: defaultFailureThreshold,
+		cooldown:  50 * time.Millisecond,
+	}
+
+	// Trip open → wait for cooldown → allow probe
+	for i := 0; i < defaultFailureThreshold; i++ {
+		cb.RecordFailure()
+	}
+	time.Sleep(60 * time.Millisecond)
+	require.True(t, cb.Allow()) // probe allowed, state → half-open
+
+	// Probe succeeds → circuit should close
+	cb.RecordSuccess()
+	assert.Equal(t, CircuitClosed, cb.State(), "successful probe should close circuit")
+
+	// Should allow deliveries again
+	assert.True(t, cb.Allow(), "closed circuit should allow deliveries")
+}
+
+func TestCircuitBreaker_ProbeFailure_ReopensCircuit(t *testing.T) {
+	t.Parallel()
+	cb := &CircuitBreaker{
+		state:     CircuitClosed,
+		threshold: defaultFailureThreshold,
+		cooldown:  50 * time.Millisecond,
+	}
+
+	// Trip open → wait for cooldown → allow probe
+	for i := 0; i < defaultFailureThreshold; i++ {
+		cb.RecordFailure()
+	}
+	time.Sleep(60 * time.Millisecond)
+	require.True(t, cb.Allow()) // probe allowed, state → half-open
+
+	// Probe fails → circuit should reopen
+	cb.RecordFailure()
+	assert.Equal(t, CircuitOpen, cb.State(), "failed probe should reopen circuit")
+	assert.False(t, cb.Allow(), "reopened circuit should reject deliveries")
+}
+
+func TestCircuitBreaker_SuccessResetsFailures(t *testing.T) {
+	t.Parallel()
+	cb := NewCircuitBreaker()
+
+	// Accumulate failures just below threshold
+	for i := 0; i < defaultFailureThreshold-1; i++ {
+		cb.RecordFailure()
+	}
+	require.Equal(t, CircuitClosed, cb.State())
+
+	// Success should reset the failure counter
+	cb.RecordSuccess()
+	assert.Equal(t, CircuitClosed, cb.State())
+
+	// Now we should need another full threshold of failures to trip
+	for i := 0; i < defaultFailureThreshold-1; i++ {
+		cb.RecordFailure()
+	}
+	assert.Equal(t, CircuitClosed, cb.State(),
+		"circuit should still be closed — success reset the counter")
+
+	// One more failure should trip it
+	cb.RecordFailure()
+	assert.Equal(t, CircuitOpen, cb.State())
+}
+
+func TestCircuitBreaker_ConcurrentAccess(t *testing.T) {
+	t.Parallel()
+	cb := NewCircuitBreaker()
+
+	const goroutines = 100
+	var wg sync.WaitGroup
+	wg.Add(goroutines * 3)
+
+	// Concurrent Allow calls
+	for i := 0; i < goroutines; i++ {
+		go func() {
+			defer wg.Done()
+			cb.Allow()
+		}()
+	}
+
+	// Concurrent RecordFailure calls
+	for i := 0; i < goroutines; i++ {
+		go func() {
+			defer wg.Done()
+			cb.RecordFailure()
+		}()
+	}
+
+	// Concurrent RecordSuccess calls
+	for i := 0; i < goroutines; i++ {
+		go func() {
+			defer wg.Done()
+			cb.RecordSuccess()
+		}()
+	}
+
+	wg.Wait()
+	// No panic or data race — the test passes if -race doesn't flag anything.
+	// State should be one of the valid states.
+	state := cb.State()
+	assert.Contains(t, []CircuitState{CircuitClosed, CircuitOpen, CircuitHalfOpen}, state,
+		"state should be valid after concurrent access")
+}
+
+func TestCircuitBreaker_CooldownRemaining_ClosedReturnsZero(t *testing.T) {
+	t.Parallel()
+	cb := NewCircuitBreaker()
+	assert.Equal(t, time.Duration(0), cb.CooldownRemaining(),
+		"closed circuit should have zero cooldown remaining")
+}
+
+func TestCircuitBreaker_CooldownRemaining_HalfOpenReturnsZero(t *testing.T) {
+	t.Parallel()
+	cb := &CircuitBreaker{
+		state:     CircuitClosed,
+		threshold: defaultFailureThreshold,
+		cooldown:  50 * time.Millisecond,
+	}
+
+	// Trip open, wait, transition to half-open
+	for i := 0; i < defaultFailureThreshold; i++ {
+		cb.RecordFailure()
+	}
+	time.Sleep(60 * time.Millisecond)
+	require.True(t, cb.Allow()) // → half-open
+
+	assert.Equal(t, time.Duration(0), cb.CooldownRemaining(),
+		"half-open circuit should have zero cooldown remaining")
+}
+
+func TestCircuitState_String(t *testing.T) {
+	t.Parallel()
+	assert.Equal(t, "closed", CircuitClosed.String())
+	assert.Equal(t, "open", CircuitOpen.String())
+	assert.Equal(t, "half-open", CircuitHalfOpen.String())
+	assert.Equal(t, "unknown", CircuitState(99).String())
+}