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/mcp/mod.rs

@@ -48,6 +48,7 @@ use rmcp::model::ReadResourceResult;
 use serde_json::Value;
 use tokio_util::sync::CancellationToken;
 
+use crate::McpProtocolMode;
 use crate::ResolvedMcpCatalog;
 use crate::connection_manager::McpConnectionSet;
 use crate::runtime::McpPublicationGate;
@@ -157,6 +158,8 @@ pub struct McpConfig {
     pub prefix_mcp_tool_names: bool,
     /// MCP servers whose model-visible tool namespaces omit the `mcp__` prefix.
     pub non_prefixed_mcp_tool_servers: Vec<String>,
+    /// Protocol compatibility policy captured when this MCP configuration is created.
+    pub protocol_mode: McpProtocolMode,
     /// Client-side elicitation capabilities advertised during MCP initialization.
     pub client_elicitation_capability: ElicitationCapability,
     /// Resolved MCP registrations keyed by logical server name.