feat(mcp): add context length guard to prevent oversized requests

* 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))

mcp/config.go

@@ -20,6 +20,7 @@ type Config struct {
 
 	// Behavior configuration
 	MaxTokens   int
+	MaxContext  int     // Model's max context window in tokens (0 = no limit)
 	Temperature float64
 	UseFullURL  bool