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

withRetry's check on a failed result write could be removed with
every test still passing, and so could six other error checks in
target_http.go. Each now has a test that fails without it: the
circuit breaker learning a failed send whose result went unrecorded,
the result write for an invalid config, building the request,
reading the response body, decoding the target config, and decoding
the stored inbound headers.

The two backoff lookups' error checks stay unpinned: without them a
failed lookup leaves a zero time, which gives the same answer, so no
test can tell the difference.

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,
 ) {