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/pkg/ctxkey/ctxkey.go

@@ -19,6 +19,9 @@ const (
 
 	// IsClaudeCodeClient 标识当前请求是否来自 Claude Code 客户端
 	IsClaudeCodeClient Key = "ctx_is_claude_code_client"
+
+	// ThinkingEnabled 标识当前请求是否开启 thinking（用于 Antigravity 最终模型名推导与模型维度限流）
+	ThinkingEnabled Key = "ctx_thinking_enabled"
 	// Group 认证后的分组信息，由 API Key 认证中间件设置
 	Group Key = "ctx_group"
 )