Protocol for tool call interceptors using handler callback pattern.
Interceptors wrap tool execution to enable request/response modification, retry logic, caching, rate limiting, and other cross-cutting concerns. Multiple interceptors compose in "onion" pattern (first is outermost).
The handler can be called multiple times (retry), skipped (caching/short-circuit), or wrapped with error handling. Each handler call is independent.
Similar to LangChain's middleware pattern but adapted for MCP remote tools.
