config: stop startup on an invalid DNS or TLS interval (closes #177)

DNSWATCHER_DNS_INTERVAL and DNSWATCHER_TLS_INTERVAL were parsed with
time.ParseDuration and silently replaced by the default when that failed,
so a value like 5 or 1d gave hourly checks with no hint why, and zero or
negative values were accepted. Both now go through parseInterval, which
returns an error naming the variable and the value, and startup stops the
same way it does for invalid targets. An unset variable still gets its
default from setupViper. The three tests that pinned the old fallback are
replaced. The README says what a valid value looks like.

Model: opus-5-5

internal/config/config_test.go

@@ -1,6 +1,7 @@
 package config_test
 
 import (
+	"strconv"
 	"testing"
 	"time"
 
@@ -113,38 +114,32 @@ func TestNew_OnlyEmptyCSVSegments(t *testing.T) {
 	assert.ErrorIs(t, err, config.ErrNoTargets)
 }
 
-func TestNew_InvalidDNSInterval_FallsBackToDefault(t *testing.T) {
-	viper.Reset()
-	t.Setenv("DNSWATCHER_TARGETS", "example.com")
-	t.Setenv("DNSWATCHER_DNS_INTERVAL", "banana")
+// TestNew_InvalidIntervalStopsStartup checks values that must stop startup;
+// TestNew_DefaultValues checks that an unset interval means the default.
+func TestNew_InvalidIntervalStopsStartup(t *testing.T) {
+	variables := []string{"DNSWATCHER_DNS_INTERVAL", "DNSWATCHER_TLS_INTERVAL"}
+	values := []string{
+		"banana", // not a duration
+		"5",      // no unit
+		"1d",     // days are not a unit time.ParseDuration knows
+		"0",      // zero
+		"-1h",    // negative
+	}
 
-	cfg, err := config.New(nil, newTestParams(t))
-	require.NoError(t, err)
-	assert.Equal(t, time.Hour, cfg.DNSInterval,
-		"invalid DNS interval should fall back to 1h default")
-}
+	for _, variable := range variables {
+		for _, value := range values {
+			t.Run(variable+"="+value, func(t *testing.T) {
+				viper.Reset()
+				t.Setenv("DNSWATCHER_TARGETS", "example.com")
+				t.Setenv(variable, value)
 
-func TestNew_InvalidTLSInterval_FallsBackToDefault(t *testing.T) {
-	viper.Reset()
-	t.Setenv("DNSWATCHER_TARGETS", "example.com")
-	t.Setenv("DNSWATCHER_TLS_INTERVAL", "notaduration")
-
-	cfg, err := config.New(nil, newTestParams(t))
-	require.NoError(t, err)
-	assert.Equal(t, 12*time.Hour, cfg.TLSInterval,
-		"invalid TLS interval should fall back to 12h default")
-}
-
-func TestNew_BothIntervalsInvalid(t *testing.T) {
-	viper.Reset()
-	t.Setenv("DNSWATCHER_TARGETS", "example.com")
-	t.Setenv("DNSWATCHER_DNS_INTERVAL", "xyz")
-	t.Setenv("DNSWATCHER_TLS_INTERVAL", "abc")
-
-	cfg, err := config.New(nil, newTestParams(t))
-	require.NoError(t, err)
-	assert.Equal(t, time.Hour, cfg.DNSInterval)
-	assert.Equal(t, 12*time.Hour, cfg.TLSInterval)
+				_, err := config.New(nil, newTestParams(t))
+				require.ErrorIs(t, err, config.ErrInvalidInterval)
+				require.ErrorContains(t, err, variable)
+				require.ErrorContains(t, err, strconv.Quote(value))
+			})
+		}
+	}
 }
 
 func TestNew_DebugEnablesDebugLogging(t *testing.T) {