feat(antigravity): comprehensive enhancements - model mapping, rate limiting, scheduling & ops

Key changes:
- Upgrade model mapping: Opus 4.5 → Opus 4.6-thinking with precise matching
- Unified rate limiting: scope-level → model-level with Redis snapshot sync
- Load-balanced scheduling by call count with smart retry mechanism
- Force cache billing support
- Model identity injection in prompts with leak prevention
- Thinking mode auto-handling (max_tokens/budget_tokens fix)
- Frontend: whitelist mode toggle, model mapping validation, status indicators
- Gemini session fallback with Redis Trie O(L) matching
- Ops: enhanced concurrency monitoring, account availability, retry logic
- Migration scripts: 049-051 for model mapping unification

backend/internal/service/gemini_multiplatform_test.go

@@ -265,6 +265,22 @@ func (m *mockGatewayCacheForGemini) DeleteSessionAccountID(ctx context.Context,
 	return nil
 }
 
+func (m *mockGatewayCacheForGemini) IncrModelCallCount(ctx context.Context, accountID int64, model string) (int64, error) {
+	return 0, nil
+}
+
+func (m *mockGatewayCacheForGemini) GetModelLoadBatch(ctx context.Context, accountIDs []int64, model string) (map[int64]*ModelLoadInfo, error) {
+	return nil, nil
+}
+
+func (m *mockGatewayCacheForGemini) FindGeminiSession(ctx context.Context, groupID int64, prefixHash, digestChain string) (uuid string, accountID int64, found bool) {
+	return "", 0, false
+}
+
+func (m *mockGatewayCacheForGemini) SaveGeminiSession(ctx context.Context, groupID int64, prefixHash, digestChain, uuid string, accountID int64) error {
+	return nil
+}
+
 // TestGeminiMessagesCompatService_SelectAccountForModelWithExclusions_GeminiPlatform 测试 Gemini 单平台选择
 func TestGeminiMessagesCompatService_SelectAccountForModelWithExclusions_GeminiPlatform(t *testing.T) {
 	ctx := context.Background()
@@ -880,7 +896,7 @@ func TestGeminiMessagesCompatService_isModelSupportedByAccount(t *testing.T) {
 		{
 			name:     "Antigravity平台-支持claude模型",
 			account:  &Account{Platform: PlatformAntigravity},
-			model:    "claude-3-5-sonnet-20241022",
+			model:    "claude-sonnet-4-5",
 			expected: true,
 		},
 		{