fix(lint): resolve go vet lostcancel and static analysis warnings from v8.0.42

- Defer context cancellation in TestCancelableConnClosesUnderlyingTransportWhenCloseRacesCancel to satisfy go vet lostcancel across all control flow paths.
- Remove the unused streamBridgeStream.close wrapper left behind after the closeWithError refactor in internal/pluginhost.
- Annotate shared "gin" string context keys in TestIssue6825SDKCallerReplayIsolation with nolint:staticcheck directives.

Closes: #6953

internal/home/cluster_balance_regression_test.go

@@ -291,6 +291,7 @@ func TestCancelableConnClosesUnderlyingTransportWhenCloseRacesCancel(t *testing.
 				}
 
 				ctx, cancel := context.WithCancel(context.Background())
+				defer cancel()
 				if mode == "pre-canceled" {
 					cancel()
 				}

internal/pluginhost/stream_bridge.go

@@ -194,10 +194,6 @@ func (s *streamBridgeStream) emit(ctx context.Context, chunk pluginapi.ExecutorS
 	return <-request.done
 }
 
-func (s *streamBridgeStream) close(errorMessage string) {
-	s.closeWithError(rpcStreamError(errorMessage, nil))
-}
-
 func (s *streamBridgeStream) closeWithError(err error) {
 	if s == nil {
 		return

internal/runtime/executor/antigravity_issue6825_test.go

@@ -396,10 +396,10 @@ func TestIssue6825SDKCallerReplayIsolation(t *testing.T) {
 				gin.SetMode(gin.TestMode)
 				ginCtxA, _ := gin.CreateTestContext(httptest.NewRecorder())
 				ginCtxA.Set("userApiKey", "downstream-api-key-A")
-				ctxA = context.WithValue(context.Background(), "gin", ginCtxA)
+				ctxA = context.WithValue(context.Background(), "gin", ginCtxA) //nolint:staticcheck -- "gin" is the shared Gin context key contract.
 				ginCtxB, _ := gin.CreateTestContext(httptest.NewRecorder())
 				ginCtxB.Set("userApiKey", "downstream-api-key-B")
-				ctxB = context.WithValue(context.Background(), "gin", ginCtxB)
+				ctxB = context.WithValue(context.Background(), "gin", ginCtxB) //nolint:staticcheck -- "gin" is the shared Gin context key contract.
 			}
 			assistantA, _ := conversation.callWithContext(ctxA, []byte(issue6825FirstRequest), optsA, scenario.enrich, metadataA)
 			assistantB, _ := conversation.callWithContext(ctxB, []byte(issue6825FirstRequest), optsB, scenario.enrich, metadataB)