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/cli/tests/daemon_startup.rs

@@ -135,7 +135,8 @@ async fn restrictive_launcher_uses_embedded_if_daemon_cannot_start() -> Result<(
             .kill_on_drop(true);
         let mut child = command.spawn()?;
         ensure!(
-            job.assign_and_resume_process(child.id().context("missing CLI pid")?)?,
+            job.assign_and_resume_process(child.id().context("missing CLI pid")?)?
+                .is_some(),
             "CLI job assignment failed"
         );
         let status = child.wait().await?;