Fix generated MCP endpoint path

[Superbil]

Summary

• restore generated MCP URLs to the shared /mcp endpoint accepted by the HTTP server
• keep agent-session routing bearer-token based
• cover the generated Codex, GitHub Copilot CLI, and Open Interpreter configurations with exact endpoint assertions

Root cause

#467 hardened the HTTP server around a bearer-authenticated, exact /mcp endpoint. #465 later preserved launch argument boundaries, but also began passing the AI Code session ID to the URL builder. That generated /mcp/<session-id>, which the server rejects with HTTP 404 before authentication or MCP protocol dispatch.

This change preserves the argument-boundary fix from #465 while restoring the endpoint contract established by #467. The bearer token still selects the registered agent session.

Codex version scope

This is not a regression introduced by Codex 0.147.0 or another recent Codex release. Codex's initial native Streamable HTTP support in 0.42.0 already consumed the configured URL verbatim, and current Codex does the same. The failing window begins in ai-code-interface.el with #465 (commit 8f60b76), which changed the generated URL from /mcp to /mcp/<session-id>.

Reference: Codex initial Streamable HTTP implementation

Relationship to existing work

• Harden the local MCP HTTP transport #458 proposed path-scoped sessions, but remains a conflicting draft and predates the bearer-token session routing merged by Harden and modernize Emacs MCP integration #467.
• Harden and modernize Emacs MCP integration #467 established the exact /mcp route with bearer authentication.
• Preserve MCP launch argument boundaries on Windows #465 unintentionally generated a session-suffixed URL while fixing launch argument boundaries.

Verification

• regression test fails against upstream main and passes with this patch
• 126 Codex, GitHub Copilot CLI, Open Interpreter, and MCP ERT tests pass in an isolated Emacs process
• check-parens and checkdoc pass for all modified files
• byte compilation passes for all modified files
• git diff --check passes

[tninja]

Thanks for the change! I’ll look into code later today.

[tninja]

@Superbil LGTM! I'm ready to merge this. Let me know if there is anything else you plan to add, or we can put it to "Ready for review" and then I'll merge it.