Add MCP 2026-07-28 discovery support (#35724)

## What changed

- Add an opt-in `mcp_2026_07_28` protocol mode while preserving the legacy
  lifecycle by default.
- Negotiate the new protocol over streamable HTTP with `server/discover`,
  including bounded responses, redirect protection, and fallback only when a
  response establishes that the endpoint is legacy-only.
- Require stdio servers to opt in with
  `CODEX_MCP_PROTOCOL_VERSION=2026-07-28`, and add a bounded local stdio
  transport for the modern lifecycle.
- Consume paginated tool, resource, and resource-template catalogs in modern
  mode, reject repeated cursors, and retain discovered server identity.
- Reconnect reusable MCP clients when their selected protocol mode changes.

## Testing

- Cover HTTP JSON and SSE discovery, legacy fallback and rejection cases,
  redirects, retries, response limits, and pagination.
- Cover local and executor stdio discovery, protocol markers, message limits,
  and legacy compatibility.

GitOrigin-RevId: f6a78816e127d2a482292d63b91c8384f1595903

codex-rs/rmcp-client/src/lib.rs

@@ -3,11 +3,14 @@ mod elicitation_client_service;
 mod executor_process_transport;
 mod http_client_adapter;
 mod in_process_transport;
+mod incoming_jsonrpc;
+mod local_stdio_transport;
 mod logging_client_handler;
 mod oauth;
 mod oauth_http_client;
 mod perform_oauth_login;
 mod program_resolver;
+mod protocol_mode;
 mod rmcp_client;
 mod startup_error;
 mod stdio_server_launcher;
@@ -36,6 +39,7 @@ pub use perform_oauth_login::perform_oauth_login;
 pub use perform_oauth_login::perform_oauth_login_return_url;
 pub use perform_oauth_login::perform_oauth_login_return_url_with_http_client;
 pub use perform_oauth_login::perform_oauth_login_silent;
+pub use protocol_mode::McpProtocolMode;
 pub use rmcp::model::ElicitationAction;
 pub use rmcp_client::Elicitation;
 pub use rmcp_client::ElicitationResponse;