Preserve environment configuration ownership (#38678)

## Why

Environment attachments can either inherit configuration from their thread or
provide their own. Later thread setting updates must refresh inherited
configuration without overwriting attachment-owned permissions and capability
roots.

## What changed

- Resolve each attachment's configuration when it is selected and retain
  whether it came from the thread or the attachment owner.
- Apply subsequent thread configuration updates only to thread-owned
  attachments, while preserving ownership across snapshots and child threads.
- Keep the resolved configuration with the environment selection so runtime
  consumers use a single canonical value.

## Testing

- Extend remote-environment coverage to verify that thread-owned permissions
  follow thread updates while owner-provided read-only permissions remain in
  effect.
- Cover configuration inheritance, attachment replacement, and owner-configured
  capability roots.

GitOrigin-RevId: fe70c4be5f151432b69bf4b141e316faa89036ae

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

@@ -448,7 +448,7 @@ impl Session {
             .chain(
                 environments
                     .turn_environments()
-                    .flat_map(|environment| environment.config.selected_capability_roots.clone()),
+                    .flat_map(|environment| environment.config().selected_capability_roots.clone()),
             )
             .enumerate()
         {