Add opt-in protocol discovery for local stdio MCP servers (#57210)

## Why

Stdio servers without `CODEX_MCP_PROTOCOL_VERSION` use the legacy handshake, even when they support MCP `2026-07-28`.

## What changed

- Add the disabled-by-default `mcp_connection_sharing` feature to enable discovery for eligible local stdio servers.
- Recover from failed or late discovery with a fresh handshake when needed, waiting for the previous helper to release resources before restarting.
- Retain a working legacy connection when an upgrade attempt fails for the same account, except on authentication errors.
- Keep tool catalogs separate by protocol and invalidate legacy catalogs when discovery negotiates the modern protocol.

## Testing

Add regression coverage for failed and late discovery, legacy fallback, protocol-specific catalog reuse, message limits, and Windows descendant cleanup.

GitOrigin-RevId: 36c675ef3b81d32fccf06ccbddda2312cfe602c9

codex-rs/features/src/lib.rs

@@ -269,6 +269,8 @@ pub enum Feature {
     Mcp20260728,
     /// Honor ordinary MCP server tool-list notifications within a turn.
     McpToolListChanged,
+    /// Discover the MCP protocol for eligible local stdio servers.
+    McpConnectionSharing,
     /// Enable MCP protocol version 2026-07-28 for the host-owned Codex Apps server.
     CodexAppsMcp20260728,
     /// Let RMCP coordinate OAuth refresh through the configured credential store.
@@ -1634,6 +1636,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::McpConnectionSharing,
+        key: "mcp_connection_sharing",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::CodexAppsMcp20260728,
         key: "codex_apps_mcp_2026_07_28",