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/executor_process_transport.rs

@@ -37,7 +37,6 @@ use rmcp::service::RoleClient;
 use rmcp::service::RxJsonRpcMessage;
 use rmcp::service::TxJsonRpcMessage;
 use rmcp::transport::Transport;
-use serde_json::from_slice;
 use serde_json::to_vec;
 use tokio::runtime::Handle;
 use tokio::sync::Semaphore;
@@ -46,6 +45,8 @@ use tracing::debug;
 use tracing::info;
 use tracing::warn;
 
+use crate::incoming_jsonrpc::deserialize_incoming_jsonrpc_message;
+
 static PROCESS_COUNTER: AtomicUsize = AtomicUsize::new(1);
 // Tool results can make valid MCP responses large, so keep the protocol
 // ceiling well above ordinary messages while still bounding hostile input.
@@ -426,7 +427,7 @@ impl ExecutorProcessTransport {
                 None => return None,
             };
             let line = Self::trim_trailing_carriage_return(line);
-            match from_slice::<RxJsonRpcMessage<RoleClient>>(&line) {
+            match deserialize_incoming_jsonrpc_message(&line) {
                 Ok(message) => return Some(message),
                 Err(error) => {
                     debug!(