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/src/state/service.rs

@@ -18,6 +18,8 @@ use arc_swap::ArcSwap;
 use codex_analytics::AnalyticsEventsClient;
 use codex_core_plugins::PluginsManager;
 use codex_exec_server::EnvironmentManager;
+use codex_extension_api::ExtensionData;
+use codex_extension_api::ExtensionRegistry;
 use codex_hooks::Hooks;
 use codex_login::AuthManager;
 use codex_mcp::McpConnectionManager;
@@ -59,6 +61,9 @@ pub(crate) struct SessionServices {
     pub(crate) skills_manager: Arc<SkillsManager>,
     pub(crate) plugins_manager: Arc<PluginsManager>,
     pub(crate) mcp_manager: Arc<McpManager>,
+    pub(crate) extensions: Arc<ExtensionRegistry<crate::config::Config>>,
+    pub(crate) session_extension_data: ExtensionData,
+    pub(crate) thread_extension_data: ExtensionData,
     pub(crate) agent_control: AgentControl,
     pub(crate) network_proxy: Option<StartedNetworkProxy>,
     pub(crate) network_approval: Arc<NetworkApprovalService>,