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/mcp-server/src/message_processor.rs

@@ -7,6 +7,7 @@ use codex_core::ThreadManager;
 use codex_core::config::Config;
 use codex_core::thread_store_from_config;
 use codex_exec_server::EnvironmentManager;
+use codex_extension_api::empty_extension_registry;
 use codex_login::AuthManager;
 use codex_login::default_client::USER_AGENT_SUFFIX;
 use codex_login::default_client::get_codex_user_agent;
@@ -68,6 +69,7 @@ impl MessageProcessor {
             auth_manager,
             SessionSource::Mcp,
             environment_manager,
+            empty_extension_registry(),
             /*analytics_events_client*/ None,
             thread_store_from_config(config.as_ref(), state_db.clone()),
             state_db.clone(),