test(hooks): add test coverage for background-notification hook

[Hungdoan565]

Add tests for the �ackground-notification hook covering the chat.message handler and event forwarding behavior.

New tests added:

• chat.message handler: skip when no background tasks, notify on completion
• event handler: session.deleted cleanup, session.idle forwarding
• dispose: cleanup on session end

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Summary by cubic

Add unit tests for the background-notification hook to verify chat message notification injection and event forwarding. Covers session.idle/error/deleted, message.updated, and session.next.tool_call.delta, and ignores unknown events.

• Dependencies
  • Sync bun.lock with package.json to bump oh-my-opencode platform binaries to 4.5.1.

^{Written for commit 83d343e. Summary will update on new commits. Review in cubic}