CBG-5141: Only fetch backup revision for fromRev for delta sync

[gregns1]

CBG-5141

• Exposed by running new added tests with bypass rev cache
• Fetch backup revision when fetching by Cv only for delta source cases. Return not found otherwise

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

Dependencies (if applicable)

• Link upstream PRs
• Update Go module dependencies when merged

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGatewayIntegration/257/ (flaky test)
• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGatewayIntegration/258/ (disable rev cache)
• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGatewayIntegration/259/ (rest only package to prove test failures fixed)
• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGatewayIntegration/260/ (rest only package with disable rev cache to prove test issues fixed)

[Copilot]

Pull request overview

This PR fixes a bug where the HLV (Hybrid Logical Vector) was incorrectly assigned when fetching backup revisions, potentially resulting in wrong current version (CV) and history for a given revision. The fix ensures that doc.HLV is only assigned when the fetched CV matches the document's current version.

Changes:

• Modified revCacheLoaderForDocumentCV to conditionally assign HLV only when the fetched CV matches the current version
• Removed incorrect CV assignment in BypassRevisionCache.GetWithCV
• Added test coverage to verify correct HLV behavior when fetching backup revisions

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
db/revision_cache_test.go	Adds test to verify HLV is correctly populated when fetching backup revisions
db/revision_cache_interface.go	Moves HLV assignment inside conditional block to only assign when CV matches current version
db/revision_cache_bypass.go	Removes incorrect CV assignment that was overwriting the fetched CV