Fix: Ensure metadata.seq >= max(doc.seq) to prevent concurrency bug in IndexedDB adapter

[Nuu-maan]

This PR fixes a concurrency bug in the IndexedDB adapter where parallel bulkDocs() transactions could result in metadata.seq being lower than the highest doc.seq, breaking replication logic.

Fix

Tracks maxSeqThisTxn during the transaction and ensures metadata.seq = Math.max(metadata.seq, maxSeqThisTxn) before writing metadata.

Test

Added integration test to test.bulk_docs.js:

• Runs 2 parallel bulkDocs()
• Verifies all doc.seq values are unique and strictly increasing
• Verifies metadata.seq >= max(doc.seq)

Fix is minimal, scoped, and backward-compatible.

[alxndrsn]

Added integration test to test.bulk_docs.js:

This file doesn't seem to have changed in the PR.

[alxndrsn]

I think this is attempting to fix the same bug as #8495

If that's correct, then my comment at #8495 (comment) is probably relevant.