fix(pds): Remove empty collections from cache on record delete

[NuroDev]

Summary

• When the last record in a collection was deleted, the collection name remained in the SQLite collections cache, so describeRepo kept reporting empty collections.
• After applying a delete, walk the repo from the ${collection}/ prefix and call a new SqliteRepoStorage.removeCollection() if no records remain in that collection.
• Adds unit tests covering the cache helper directly and the rpcDeleteRecord integration (last-record removal vs. records-remaining retention).

Test plan

• pnpm --filter @getcirrus/pds test passes (249 tests)
• Manually verify describeRepo stops listing a collection after deleting its final record

🤖 Generated with Claude Code

[ascorbic]

Thanks!

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commit: 9e2d8f5

[ascorbic]

Actually, I realise this should also be applied to rpcApplyWrites, because that can also be used to delete records (and is in fact the more common way)

[NuroDev]

Actually, I realise this should also be applied to rpcApplyWrites, because that can also be used to delete records (and is in fact the more common way)

Ah okay, good point. Pushed a new commit that includes this change + tests.