refactor: optimize project_id fill to lightweight approach

- Replace heavy RefreshAccountToken with lightweight tryFillProjectID
  (loadCodeAssist → onboardUser → fallback), consistent with
  Antigravity-Manager's behavior
- Add sync.Map cooldown/dedup (60s) to prevent repeated fill attempts
- Add fallback project_id "bamboo-precept-lgxtn" matching AM
- Extract mergeCredentials helper to eliminate duplication
- Use slog structured logging instead of log.Printf
- Fix time.Sleep in OnboardUser to context-aware select
- Fix strings.NewReader(string(bodyBytes)) → bytes.NewReader(bodyBytes)
- Remove redundant tc := tc in test (Go 1.22+)
- Add nil guard in persistProjectID for test safety

backend/internal/service/antigravity_oauth_service_test.go

@@ -51,7 +51,6 @@ func TestResolveDefaultTierID(t *testing.T) {
 	}
 
 	for _, tc := range tests {
-		tc := tc
 		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()