Feature Request: Support for MCP 241105 Spec in Envelope Stateful Session Extension

Title: Support MCP 241105 Spec in Envelope Stateful Session Extension

Description:

The current envelope stateful session extension (introduced in PR #39004) only supports the MCP 250326 spec (header-based session tracking).

However, the MCP 241105 spec is not supported. This spec requires Envoy to decode the initial SSE event in the upstream response, extract and update the session ID, and manage the session context accordingly.

Supporting MCP 241105 would enable compatibility with upstream servers and clients that rely on this protocol for session management, especially for SSE-based applications.

Please consider adding native support for MCP 241105 in the envelope stateful session extension, so that Envoy can handle session context as required by this spec, without relying on Lua scripts or external workarounds.

Relevant Links:

• PR #39004: stateful session: new envelope stateful session extension
• [MCP 241105 Spec] (https://modelcontextprotocol.io/specification/2024-11-05/basic/transports#http-with-sse)