feat: add token usage contributor hook

[jif-oai]

Why

Extensions need a stable place to observe token accounting after Codex folds model-provider usage into the session's cached TokenUsageInfo. Without a contributor hook, extension-owned features that need last-turn or cumulative token usage have to duplicate session plumbing or infer state from client-facing TokenCount notifications.

What changed

• Added TokenUsageContributor to codex-extension-api, passing session/thread ExtensionData, ThreadId, turn id, and the current TokenUsageInfo.
• Added registry builder/storage support for token-usage contributors.
• Invoked registered contributors from Session::record_token_usage_info after the session token cache is updated and before the client TokenCount notification is emitted.

Testing

• Added record_token_usage_info_notifies_extension_contributors, covering cumulative token usage updates and access to both extension stores.