Add direct tests for idle-close and reply-token flows

[NahuelGenchi]

Pull Request Description

Summary

• Add direct idle-close job tests for stale threads, fresh threads, and the onCloseThread hook.
• Add reply-token cache tests for inbound webhook capture, line.ack_with_reply_token, expired tokens, and reply-token GC.
• Keep the change tests-only with no production code changes.

Closes #8

Validation

• npm test
• npm run typecheck
• npm run build

[harleyjj]

Thank you for tackling this! I'd like to request some changes:

BLOCKING: tests/reply-token.test.ts misses the production index invariant for reply-token GC.

Concrete problem: the inbound test proves that comment metadata exists after webhook processing (tests/reply-token.test.ts:190-196), but it never asserts that the resulting commentId was added to STATE_KEYS.commentMetaIndex. The GC test then hand-seeds that same index (tests/reply-token.test.ts:250-255) before running reply-token-gc, so the suite would still pass if the real inbound path stored reply-token metadata but stopped indexing it. That is the production failure mode that matters: unindexed inbound tokens would never be swept by reply-token-gc, even though these tests stay green.

Proposed fix: after the webhook/process-event path captures commentId, assert harness.getState(replyTokenIndexScope()) contains { companyId: COMPANY_ID, commentId }. For stronger coverage, make the GC test consume an entry created through the inbound webhook path, then age that captured meta before running JOB_KEYS.replyTokenGc, so the GC assertion depends on the same index path production uses.