mcp: Implement stateless server (SEP-2275)

[guglielmo-san]

Description

This PR lays the foundational server-side groundwork for the >= 2026-06-30 sessionless and stateless feature introduced by SEP-2575 and SEP-2567, tracked in design/stateless.md.

SEP-2575: Stateless MCP

• Per-request protocol detection in ServerSession.handle()
  Unmarshal _meta from the raw JSON-RPC params determines whether each request follows the new protocol.

• Per-request typed accessors on ServerRequest[P]

  func (r *ServerRequest[P]) ProtocolVersion() string
  func (r *ServerRequest[P]) ClientInfo() *Implementation
  func (r *ServerRequest[P]) ClientCapabilities() *ClientCapabilities

• Reject client->server initialize, initialized and ping for new-protocol requests

• Per-request _meta field name constants
  Three constants for the wire-protocol field names (MetaKeyProtocolVersion, MetaKeyClientInfo, MetaKeyClientCapabilities)

• Stop synthesizing fake InitializeParams for new-protocol requests

Fixes: #966