Allow environment readiness updates in place (#35875)

## What changed

- Add `EnvironmentManager::publish_ready_info` to update the selected capability
  roots for an existing environment without replacing it.
- Store readiness as an atomically replaceable snapshot so repeated publications
  expose the latest roots while deferred completion still controls connection
  readiness.
- Validate published roots and reject updates for missing environments.

## Testing

- Cover publication, replacement, repeated and invalid updates, and interaction
  with deferred environment completion.

GitOrigin-RevId: 1477f60e1319f840e6b33703ef2af052713779fd

codex-rs/core/src/session/mcp.rs

@@ -323,6 +323,7 @@ impl Session {
             .services
             .selected_capability_roots
             .iter()
+            .cloned()
             .chain(
                 environments
                     .turn_environments()
@@ -352,7 +353,7 @@ impl Session {
                 ready_environment_root_count += 1;
             }
             root_locations_by_id.insert(root.id.clone(), root.location.clone());
-            selected_capability_roots.push(root.clone());
+            selected_capability_roots.push(root);
         }
         self.services
             .turn_environments