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/account_test_service.go

@@ -245,19 +245,17 @@ func (s *AccountTestService) testClaudeAccountConnection(c *gin.Context, account
 	// Set common headers
 	req.Header.Set("Content-Type", "application/json")
 	req.Header.Set("anthropic-version", "2023-06-01")
+	req.Header.Set("anthropic-beta", claude.DefaultBetaHeader)
 
-	// Set authentication header and beta header based on account type
+	// Apply Claude Code client headers
+	for key, value := range claude.DefaultHeaders {
+		req.Header.Set(key, value)
+	}
+
+	// Set authentication header
 	if useBearer {
-		// OAuth 账号使用完整的 Claude Code beta header
-		req.Header.Set("anthropic-beta", claude.DefaultBetaHeader)
 		req.Header.Set("Authorization", "Bearer "+authToken)
-		// Apply Claude Code client headers for OAuth
-		for key, value := range claude.DefaultHeaders {
-			req.Header.Set(key, value)
-		}
 	} else {
-		// API Key 账号使用简化的 beta header（不含 oauth）
-		req.Header.Set("anthropic-beta", claude.APIKeyBetaHeader)
 		req.Header.Set("x-api-key", authToken)
 	}