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/tools/spec_plan.rs

@@ -954,7 +954,7 @@ fn tool_environment_mode(environments: &TurnEnvironmentSnapshot) -> ToolEnvironm
 fn any_environment_allows_login_shell(environments: &TurnEnvironmentSnapshot) -> bool {
     environments
         .turn_environments()
-        .any(|environment| environment.config.allow_login_shell)
+        .any(|environment| environment.config().allow_login_shell)
 }
 
 #[instrument(level = "trace", skip_all)]