feat(http): serve v2 over Streamable HTTP and WebSocket

[odellus]

Summary

The Streamable HTTP + WebSocket transports predate the experimental v2 runtime and were hard-wired to the v1 AgentSideConnection and its method table. This completes the port so both transports are protocol-agnostic, matching the TypeScript reference (AcpServer takes an AgentConnector).

What changed

• AcpServer / handle_websocket accept an AgentProtocolRouter, which negotiates v1 or v2 from the first initialize and attaches the matching runtime (v1 AgentFactory keeps working unchanged).
• AgentProtocolConnection gains a public listen() so stdio, Streamable HTTP, and WebSocket drive it uniformly.
• session/resume joins session/load as a replay method (LOAD_SESSION_METHODS), so v2 resume routes replayed history over the connection stream exactly like v1 load.
• V2AgentSideConnection.attach now calls on_connect, matching __init__ and v1's attach. (v2 agents that learn their connection via on_connect — e.g. crow-cli v2 — otherwise never receive it through the router path.)

Testing

• New tests/http/test_v2_loopback.py: v2 over Streamable HTTP, v2 over WebSocket, v2 session/resume replay, and a dual-stack test serving v1 + v2 clients on one port.
• Full suite: 361 passed, 1 skipped.
• ruff check, ruff format --check, ty check, deptry src, uv lock --locked all pass.

Checklist

• Conventional Commit title.
• Tests cover the change.
• make test green (pytest --doctest-modules).
• User-facing transport behaviour documented in the module docstrings (no schema regeneration involved).