transports: Revisions on streamable-http/ws GET streams (#1124)

* transports: Revisions on streamable-http/ws GET streams

Server->client messages are now delivered over two long-lived GET SSE
streams per connection:

  1. Connection-scoped stream (GET with Acp-Connection-Id) carries
     connection-level messages, including responses to session/new and
     session/load which are produced before the client has a sessionId.

  2. Session-scoped stream (GET with Acp-Connection-Id + Acp-Session-Id)
     carries session updates, request_permission, and responses to
     session-scoped POSTs.

Routing is now done by HTTP header rather than by inspecting the
sessionId in every JSON-RPC payload, and per-session streams have
independent lifetimes (open/close/resume) without disturbing other
sessions on the same connection.

Updates the elevator pitch, HTTP transport characteristics, routing
table, identity model, message flow diagram (with per-stream C/S
annotations), content negotiation rules, unified endpoint routing,
connection/session struct, MCP comparison, deviations, FAQ, and
revision history.

* Format streamable-http-websocket-transport.mdx with prettier

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Anna.Zhdan <anna.zhdan@jetbrains.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 3 people