feat: add config-change extension contributor (#22488)

## Why

Extensions can observe thread and turn lifecycle events today, but there
was no single host-owned hook for changes to the effective thread
configuration. That makes features that need to react to model,
permission, or tool-suggest updates either depend on individual mutation
paths or risk going stale after runtime config refreshes.

This adds a typed config-change contributor so extension-owned state can
stay synchronized with the effective thread config while the host
remains responsible for deciding when config changed.

## What Changed

- Added `ConfigContributor<C>` to `codex_extension_api`, with
before/after immutable snapshots of the effective config plus
session/thread extension stores.
- Added registry builder/accessor support through `config_contributor`
and `config_contributors`.
- Emits config-change callbacks after committed updates from session
settings, per-turn setting updates, and `refresh_runtime_config`.
- Builds effective config snapshots only when config contributors are
registered, and suppresses no-op callbacks when the before/after
snapshots are equal.
- Added a core session regression test that verifies contributors
observe both model changes and user-layer runtime config changes,
including access to session and thread extension stores.

## Validation

Added `config_change_contributor_observes_effective_config_changes` in
`codex-rs/core/src/session/tests.rs` to cover the new contributor path.

codex-rs/ext/extension-api/src/contributors.rs

@@ -67,6 +67,23 @@ pub trait TurnLifecycleContributor: Send + Sync {
     fn on_turn_abort(&self, _input: TurnAbortInput<'_>) {}
 }
 
+/// Contributor for host-owned configuration changes.
+///
+/// Implementations should treat the supplied values as immutable before/after
+/// snapshots of the effective thread configuration.
+pub trait ConfigContributor<C>: Send + Sync {
+    /// Called after the host commits a changed thread configuration.
+    fn on_config_changed(
+        &self,
+        _session_store: &ExtensionData,
+        _thread_store: &ExtensionData,
+        _thread_id: ThreadId,
+        _previous_config: &C,
+        _new_config: &C,
+    ) {
+    }
+}
+
 /// Contributor for token usage checkpoints reported by the model provider.
 ///
 /// Implementations should keep this callback cheap. The host calls it after