feat: use stateless MCP discovery

[Sam123ben]

Summary

• use the official SDK server/discover path for AgentGateway MCP operations
• require the MCP 2.1.1 stateless contract and record its dependency policy
• document the production wire behavior while retaining the transport-neutral compatibility interface

Validation

• make check
• 9,428 tests passed, 2 skipped; 96.55% coverage
• Ruff, import-linter, mypy strict, Pyright, dependency/admission, docs, API, replay, release, and schema gates passed