extension: wire extension registries into sessions (#21737)

## Why

[#21736](https://github.com/openai/codex/pull/21736) introduces the
typed extension API, but the runtime does not yet carry a registry
through thread/session startup or give contributors host-owned stores to
read from. This PR wires that host-side path so later feature migrations
can move product-specific behavior behind typed contributions without
adding another bespoke seam directly to `codex-core`.

## What changed

- Thread `ExtensionRegistry<Config>` through `ThreadManager`,
`CodexSpawnArgs`, `Session`, and sub-agent spawn paths.
- Wire `ThreadStartContributor` and `ContextContributor`
- Expose the small supporting surface needed by non-core callers that
construct threads directly, including `empty_extension_registry()`
through `codex-core-api`.

This PR lands the host plumbing only: the app-server registry is still
empty, and concrete feature migrations are intended to follow
separately.

codex-rs/core-api/src/lib.rs

@@ -45,6 +45,7 @@ pub use codex_core::skills::SkillsManager;
 pub use codex_core::thread_store_from_config;
 pub use codex_exec_server::EnvironmentManager;
 pub use codex_exec_server::ExecServerRuntimePaths;
+pub use codex_extension_api::empty_extension_registry;
 pub use codex_features::Feature;
 pub use codex_features::Features;
 pub use codex_login::AuthManager;