Cache executor metadata from initialization (#40343)

## What changed

- Include `environmentInfo` in the exec-server `initialize` response and seed the Rust client's metadata cache from it.
- Fall back to one lazy `environment/info` request when connecting to older servers that omit initialization metadata, then share the cached result across client clones and session recovery.
- Keep app-server `environment/info` requests uncached so they continue to probe the executor.

## Testing

- Cover initialization metadata, the legacy-server fallback, shared caching, accepted WebSocket environments, and repeated uncached app-server probes.

GitOrigin-RevId: 380132d9db183998d73e8c6b0954e4d70c7e8406

codex-rs/exec-server/src/server/handler.rs

@@ -156,7 +156,10 @@ impl ExecServerHandler {
             .session
             .lock()
             .unwrap_or_else(std::sync::PoisonError::into_inner) = Some(session);
-        Ok(InitializeResponse { session_id })
+        Ok(InitializeResponse {
+            session_id,
+            environment_info: Some(EnvironmentInfo::local()),
+        })
     }
 
     pub(crate) fn initialized(&self) -> Result<(), String> {