Pin the HTTP target's unpinned error checks (closes #285)

Seven error checks in internal/delivery/target_http.go could be removed without any test noticing, among them withRetry's check on writing the delivery result, the branch that leaves a sent delivery retrying and recoverable when its bookkeeping write fails. Each now fails a test when removed. The "send succeeded" case starts from a tripped circuit breaker, so a probe whose send succeeds but whose result write fails must still close the breaker. The checks in remainingBackoff and backoffElapsed stay unpinned: removing them gives the same answer, and they state a rule a reader needs. Test change only.

Model: opus-5-5

internal/delivery/engine_test.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"log/slog"
 	"net/http"
 	"net/http/httptest"
@@ -1056,6 +1057,21 @@ func TestParseHTTPConfig_MissingURL(t *testing.T) {
 	)
 }
 
+func TestParseHTTPConfig_Undecodable(t *testing.T) {
+	t.Parallel()
+
+	e := testEngine(t, 1)
+
+	_, err := e.ExportParseHTTPConfig(
+		`{"url":"https://example.com/hook","timeout":"soon"}`,
+	)
+
+	assert.Error(t, err,
+		"config that does not decode should return error, "+
+			"even when the part that did names a URL",
+	)
+}
+
 func TestScheduleRetry_SendsToRetryChannel(
 	t *testing.T,
 ) {
@@ -1241,6 +1257,33 @@ func TestDoHTTPRequest_ForwardsHeaders(t *testing.T) {
 	)
 }
 
+// A response that ends before the length it announced is an error, not
+// a short body.
+func TestDoHTTPRequest_CutShortResponseIsAnError(t *testing.T) {
+	t.Parallel()
+
+	ts := httptest.NewServer(
+		http.HandlerFunc(
+			func(w http.ResponseWriter, _ *http.Request) {
+				w.Header().Set("Content-Length", "100")
+				_, _ = w.Write([]byte("cut short"))
+			},
+		),
+	)
+	defer ts.Close()
+
+	e := testEngine(t, 1)
+
+	_, body, _, err := e.ExportDoHTTPRequest(
+		context.TODO(),
+		&delivery.HTTPTargetConfig{URL: ts.URL},
+		&database.Event{},
+	)
+
+	require.ErrorIs(t, err, io.ErrUnexpectedEOF)
+	assert.Empty(t, body)
+}
+
 // The event's stored inbound headers carry the same Content-Type the
 // receiver saved as the event's ContentType, so a delivery could send
 // it twice. It must go out exactly once, with a Content-Type configured
@@ -1317,6 +1360,34 @@ func TestApplyRequestHeaders_SendsOneContentType(t *testing.T) {
 	}
 }
 
+// Stored inbound headers that do not decode forward nothing, not the
+// part of them that happened to decode.
+func TestApplyRequestHeaders_UndecodableInboundForwardsNothing(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	req, err := http.NewRequestWithContext(
+		context.Background(),
+		http.MethodPost,
+		"https://target.example.com/hook",
+		http.NoBody,
+	)
+	require.NoError(t, err)
+
+	names := delivery.ExportApplyRequestHeaders(
+		req,
+		&database.Event{
+			Headers: `{"X-Custom":["value1"],"X-Broken":"not a list"}`,
+		},
+		&delivery.HTTPTargetConfig{},
+		"webhooker/dev",
+	)
+
+	assert.Empty(t, names)
+	assert.Empty(t, req.Header.Get("X-Custom"))
+}
+
 func TestProcessDelivery_RoutesToCorrectHandler(
 	t *testing.T,
 ) {