[WIP] RFD-based ACP HTTP and WS web transport implementation

[federicociner]

Implements the remote transport from the RFD (Streamable HTTP + WebSocket), giving ACP a standard remote transport alongside the existing stdio path. Based on the RFD for web transport and reference implementations that already exist in the Rust and TypeScript SDKs.

What's included

• Transport seam (_transport.py): message-level Transport protocol; stdio re-expressed as NdjsonTransport with zero behavior change. Connection and connect_to_agent now accept a Transport in addition to byte streams.
• Streamable HTTP (http/): POST for client→server (202, except initialize which returns 200 + Acp-Connection-Id), long-lived SSE GET streams for server→client (connection- and session-scoped), DELETE to terminate. Framework-agnostic core + thin ASGI adapter.
• WebSocket (ws/): full-duplex upgrade on the same endpoint, multiplexing all server→client traffic onto one socket.
• Client transports: create_http_stream() and create_websocket_stream().

Reliability hardening

• Backpressure: OutboundStream awaits on a full buffer instead of silently
  dropping messages (a dropped response would hang the peer permanently).
• SSE keepalive: idle streams emit periodic keepalive comments so intermediaries don't time out healthy connections.
• Disconnect surfacing: HTTP client surfaces EOF when the connection-scoped SSE stream ends, so pending requests reject instead of hanging.
• initialize cleanup: failed/timed-out initialize tears down the connection (504/500) instead of leaking it.
• WebSocket cookies: client sends/stores affinity cookies.

[federicociner]

I am marking this PR as WIP until I have been able to do E2E testing on our internal infrastructure; we have an agentic runtime platform internally within at Atlassian that exposes an ACP over HTTP endpoint, so we can stress test the Python SDK implementation against this server.

I will update the PR once I've done E2E testing.