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/streamable_http_retry_tests.rs

@@ -10,8 +10,9 @@ use crate::http_client_adapter::StreamableHttpClientAdapterError;
 use super::*;
 
 #[test]
-fn retryable_initialize_error_includes_initialized_notification_context() {
+fn retryable_initialize_error_includes_discovery_and_initialized_notification_context() {
     let contexts = [
+        "send discover request",
         "send initialize request",
         "send initialized notification",
         "receive initialize response",
@@ -21,7 +22,7 @@ fn retryable_initialize_error_includes_initialized_notification_context() {
         contexts.map(|context| {
             RmcpClient::is_retryable_client_initialize_error(&retryable_initialize_error(context))
         }),
-        [true, true, false],
+        [true, true, true, false],
     );
 }