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/app-server/src/request_processors/config_processor.rs

@@ -72,6 +72,7 @@ const SUPPORTED_EXPERIMENTAL_FEATURE_ENABLEMENT: &[&str] = &[
     BACKGROUND_PAGINATED_ROLLOUT_MIGRATION_FEATURE,
     "codex_apps_mcp_2026_07_28",
     "mcp_2026_07_28",
+    "mcp_connection_sharing",
     "memories",
     "mentions_v2",
     "remote_control",