feat(mcp): Add McpServerInterceptor with scoped hooks

[ClayGifford1]

Summary

Server interceptor hooks:

• Adds McpServerInterceptor interface with scoped read/modify hooks at specific pipeline stages, following the ClientInterceptor pattern
• Hooks: readBefore/modifyBefore and readAfter/modifyAfter for both execution-level and tool-call-level stages
• Hook data via McpExecutionHook and McpToolCallHook (typed context, request, protocol version, tool metadata)
• McpServerInterceptor.chain() for composing multiple interceptors, with error handling matching ClientInterceptorChain (swapError, last-write-wins for read hooks, immediate propagation for modify hooks)
• Single interceptor setter on McpService.Builder.interceptor() and McpServerBuilder.interceptor()
• Zero-cost NOOP fast path when no interceptor is configured

Extensible proxy transport:

• Makes McpServerProxy.rpc(), start(), shutdown() protected to allow custom proxy transport implementations outside the package

Motivation

There's currently no way to observe, augment, or short-circuit requests at the McpService level. Consumers using McpService directly can wrap it, but that customization is lost when using McpServer, which creates and owns the McpService instance internally. Placing the interceptor on McpService.Builder solves both cases — direct McpService users add interceptors at construction, and McpServerBuilder plumbs them through.

The hook-based approach follows the established ClientInterceptor pattern — scoped hooks with typed context rather than a generic request handler chain. McpService owns when hooks fire, so interceptors never need to understand async dispatch internals.

Separately, custom MCP proxy transports beyond the provided stdio and HTTP implementations are not possible today because McpServerProxy's abstract methods are package-private. Making them protected allows consumers to implement their own proxy transports.