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/openai_gateway_service_test.go

@@ -204,6 +204,22 @@ func (c *stubGatewayCache) DeleteSessionAccountID(ctx context.Context, groupID i
 	return nil
 }
 
+func (c *stubGatewayCache) IncrModelCallCount(ctx context.Context, accountID int64, model string) (int64, error) {
+	return 0, nil
+}
+
+func (c *stubGatewayCache) GetModelLoadBatch(ctx context.Context, accountIDs []int64, model string) (map[int64]*ModelLoadInfo, error) {
+	return nil, nil
+}
+
+func (c *stubGatewayCache) FindGeminiSession(ctx context.Context, groupID int64, prefixHash, digestChain string) (uuid string, accountID int64, found bool) {
+	return "", 0, false
+}
+
+func (c *stubGatewayCache) SaveGeminiSession(ctx context.Context, groupID int64, prefixHash, digestChain, uuid string, accountID int64) error {
+	return nil
+}
+
 func TestOpenAISelectAccountWithLoadAwareness_FiltersUnschedulable(t *testing.T) {
 	now := time.Now()
 	resetAt := now.Add(10 * time.Minute)