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/url_mask_test.go

@@ -179,6 +179,27 @@ func TestDoHTTPRequest_TransportErrorMasksURL(t *testing.T) {
 	)
 }
 
+// TestDoHTTPRequest_UnparsableURLIsMasked is the same for an HTTP
+// target URL that no request can be built from.
+func TestDoHTTPRequest_UnparsableURLIsMasked(t *testing.T) {
+	t.Parallel()
+
+	e := testEngine(t, 1)
+
+	statusCode, _, _, reqErr := e.ExportDoHTTPRequest(
+		context.TODO(),
+		&delivery.HTTPTargetConfig{
+			URL: "https://hooks.example.com" + maskSecretPath + "\n",
+		},
+		&database.Event{},
+	)
+	require.Error(t, reqErr)
+	assert.Zero(t, statusCode)
+
+	assertNoCredential(t, reqErr.Error())
+	assert.Contains(t, reqErr.Error(), "invalid control character")
+}
+
 // TestValidateTargetURL_UnparsableURLIsMasked proves the SSRF
 // validator's error does not carry the submitted URL, which
 // the handler both logs and shows.