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/codex-mcp/src/lib.rs

@@ -1,5 +1,6 @@
 pub use binding::McpBinding;
 pub use binding::PreparedMcpCall;
+pub use codex_rmcp_client::McpProtocolMode;
 pub use connection_manager::tool_is_model_visible;
 pub use elicitation::ElicitationLifecycle;
 pub use elicitation::ElicitationReviewRequest;
@@ -91,6 +92,7 @@ pub(crate) mod connection_manager;
 pub(crate) mod elicitation;
 pub(crate) mod mcp;
 mod openai_docs_source_attribution;
+mod pagination;
 mod plugin_config;
 mod resource_client;
 pub(crate) mod rmcp_client;