* feat: add X-Client-ID header for claw402 monitoring
* feat(mcp): add context length guard to prevent oversized requests
- Add MaxContext field to Config (default 0 = no limit)
- Add WithMaxContext() option for setting model context limits
- Add context_guard.go: token estimation + message truncation
- Integrate guard into both BuildMCPRequestBody and BuildRequestBodyFromRequest
- Support both map[string]string and map[string]any message formats
- Truncates oldest non-system messages when estimated tokens exceed limit
- Always preserves system messages and keeps at least 1 non-system message
- Logs warning when truncation occurs for debugging
Usage: mcp.NewDeepSeekClient(mcp.WithMaxContext(131072))
