Move plugin skill snapshot integration tests into core (#37169)

## What changed

- Move the integration test for reusing plugin-manager skill snapshots into
  `codex-core`, which owns both the plugin manager and host skills service
  integration.
- Keep the `codex-core-plugins` test focused on resolving local and remote
  plugin identities on effective skill roots, and remove its skills-extension
  dev dependency.
- Use the skills extension's `HostSkillsSnapshot` type in its world-state
  catalog implementation.

## Testing

- Verify the host skills service reuses the plugin manager's parsed snapshot
  after the underlying skill file changes, while preserving plugin identity
  metadata.

GitOrigin-RevId: afc8a0ebccb629360c59a4546bdb9a134c74da4f

codex-rs/core/src/plugins/mod.rs

@@ -3,6 +3,9 @@ mod injection;
 mod mentions;
 mod render;
 #[cfg(test)]
+#[path = "skill_snapshot_tests.rs"]
+mod skill_snapshot_tests;
+#[cfg(test)]
 pub(crate) mod test_support;
 
 pub(crate) use codex_plugin::PluginCapabilitySummary;