test(feishu): bound webhook dispatch wait to 2s, not test timeout

larkevents_test used `<-t.Context().Done()` as the timeout branch in
the dispatch wait select. t.Context() is only canceled when the test
function returns, so a missing dispatch call would deadlock the test
for the full default go test timeout (10 min) before failing — a real
CI hazard, discovered when an unrelated HMAC-verify experiment
prevented dispatch and the CI run sat idle for 10 minutes.

Replace with `time.After(2*time.Second)` via a named constant, plus a
comment warning future readers not to revert the pattern.

internal/channels/feishu/larkevents_test.go

@@ -10,8 +10,18 @@ import (
 	"net/http/httptest"
 	"strings"
 	"testing"
+	"time"
 )
 
+// dispatchWaitTimeout bounds how long a webhook dispatch test will block on
+// the onMessage channel before failing. Kept short (2s) so a regression in
+// the dispatch path cannot hang CI for the full go test default (10 min).
+// Do NOT replace with t.Context().Done() — that context is only canceled
+// after the test returns, which creates a deadlock if the channel never
+// receives. This bit Wave C when an unrelated HMAC patch suppressed
+// dispatch and the test ran for 10 minutes before timing out.
+const dispatchWaitTimeout = 2 * time.Second
+
 // --- helpers ---
 
 // encryptEventPayload encrypts a JSON payload using the same AES-CBC scheme
@@ -164,7 +174,7 @@ func TestWebhookHandler_TokenMatch_Dispatches(t *testing.T) {
 		if e == nil {
 			t.Error("dispatched event was nil")
 		}
-	case <-t.Context().Done():
+	case <-time.After(dispatchWaitTimeout):
 		t.Error("timeout waiting for dispatched event")
 	}
 }
@@ -257,7 +267,7 @@ func TestWebhookHandler_EncryptedEvent_Decrypted(t *testing.T) {
 		if e == nil {
 			t.Error("dispatched event was nil")
 		}
-	case <-t.Context().Done():
+	case <-time.After(dispatchWaitTimeout):
 		t.Error("timeout waiting for dispatched encrypted event")
 	}
 }