fix(logger): resolve Windows pipe fsync deadlock in tests

Remove Sync() calls in logger tests that use os.Pipe for stdout/stderr
capture. On Windows, FlushFileBuffers on pipe handles blocks
indefinitely, causing test timeouts (600s+).

Also use os.MkdirTemp instead of t.TempDir to avoid cleanup failures
when lumberjack still holds file handles.

backend/internal/pkg/logger/logger_test.go

@@ -10,7 +10,13 @@ import (
 )
 
 func TestInit_DualOutput(t *testing.T) {
-	tmpDir := t.TempDir()
+	// Use os.MkdirTemp instead of t.TempDir to avoid cleanup failures
+	// when lumberjack holds file handles on Windows.
+	tmpDir, err := os.MkdirTemp("", "logger-test-*")
+	if err != nil {
+		t.Fatalf("create temp dir: %v", err)
+	}
+	t.Cleanup(func() { _ = os.RemoveAll(tmpDir) })
 	logPath := filepath.Join(tmpDir, "logs", "sub2api.log")
 
 	origStdout := os.Stdout
@@ -57,7 +63,9 @@ func TestInit_DualOutput(t *testing.T) {
 
 	L().Info("dual-output-info")
 	L().Warn("dual-output-warn")
-	Sync()
+
+	// Skip Sync() — on Windows, fsync on pipes deadlocks (FlushFileBuffers).
+	// The log data is already in the pipe buffer; closing writers is sufficient.
 
 	_ = stdoutW.Close()
 	_ = stderrW.Close()
@@ -166,7 +174,9 @@ func TestInit_CallerShouldPointToCallsite(t *testing.T) {
 	}
 
 	L().Info("caller-check")
-	Sync()
+	// Skip Sync() — on Windows, fsync on pipes deadlocks (FlushFileBuffers).
+	os.Stdout = origStdout
+	os.Stderr = origStderr
 	_ = stdoutW.Close()
 	logBytes, _ := io.ReadAll(stdoutR)
 

backend/internal/pkg/logger/stdlog_bridge_test.go

@@ -77,7 +77,7 @@ func TestStdLogBridgeRoutesLevels(t *testing.T) {
 	log.Printf("service started")
 	log.Printf("Warning: queue full")
 	log.Printf("Forward request failed: timeout")
-	Sync()
+	// Skip Sync() — on Windows, fsync on pipes deadlocks (FlushFileBuffers).
 
 	_ = stdoutW.Close()
 	_ = stderrW.Close()
@@ -139,7 +139,7 @@ func TestLegacyPrintfRoutesLevels(t *testing.T) {
 	LegacyPrintf("service.test", "request started")
 	LegacyPrintf("service.test", "Warning: queue full")
 	LegacyPrintf("service.test", "forward failed: timeout")
-	Sync()
+	// Skip Sync() — on Windows, fsync on pipes deadlocks (FlushFileBuffers).
 
 	_ = stdoutW.Close()
 	_ = stderrW.Close()