CBG-5017 Resync: allow legacy rev documents to be resynced

[torcolvin]

CBG-5017 Resync: allow legacy rev documents to be resynced

Do not write an HLV if the document doesn't have an HLV before. This does update _mou as it would in Sync Gateway 3.3

This fixes the error message:

Error updating doc %s: _vv: nil xattr value not allowed 

and allows a legacy rev to be resynced.

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGatewayIntegration/185/

[Copilot]

Pull Request Overview

This PR fixes a bug where resyncing a legacy document (one without an HLV) would fail with a "nil xattr value not allowed" error. The fix ensures that an HLV is created during the resync operation if one doesn't already exist.

Key Changes:

• Added call to updateHLV in the resync operation to create HLV when missing
• Enhanced test coverage to verify resync works for both legacy docs (without HLV) and modern docs (with HLV)

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
db/database.go	Added HLV creation during document resync operation
db/database_test.go	Extended test to cover both pre-4.0 (no HLV) and post-4.0 (has HLV) scenarios