fix(tests): adapt TestMain_DefaultStartupGracefulShutdown_Subprocess to Go 1.26.0 signal handling

- Increased SIGTERM signal timeout from 500ms to 1000ms
- Go 1.26.0 changed signal delivery timing on Linux
- Test now passes reliably with adequate startup grace period

Related to Go 1.26.0 upgrade (commit dc40102a)

backend/cmd/api/main_test.go

@@ -289,7 +289,8 @@ func TestMain_DefaultStartupGracefulShutdown_Subprocess(t *testing.T) {
 		os.Args = []string{"charon"}
 
 		go func() {
-			time.Sleep(500 * time.Millisecond)
+			// Increased from 500ms to 1000ms for Go 1.26.0 signal handling changes
+			time.Sleep(1000 * time.Millisecond)
 			process, err := os.FindProcess(os.Getpid())
 			if err == nil {
 				_ = process.Signal(syscall.SIGTERM)