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/lib.rs

@@ -15,6 +15,7 @@ pub use codex_tools::ToolSpec;
 pub use codex_tools::parse_tool_input_schema;
 pub use contributors::ApprovalReviewContributor;
 pub use contributors::ApprovalReviewFuture;
+pub use contributors::ConfigContributor;
 pub use contributors::ContextContributor;
 pub use contributors::ExtensionToolExecutor;
 pub use contributors::ExtensionToolFuture;