CBG-4907: fix for panic in CheckProposedVersion for legacy revision case

[gregns1]

CBG-4907

• If we have no local HLV and parentRev doesn't match local current rev, we should return conflict with current rev as conflict rev for proposeChangesResponse
• Test repros the panic if you take out my fix in CheckProposedVersion and run the test.
• Test is skipped, given there is no conflict resolution for rev tree Ids in the blip tester client, I've had a go a doing this but running out of time to get it done properly and its not something I would want to rush, I will file a ticket to get this implemented and remove the skip for this test (I don't think this needs to go into 4.0.0 though)

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/0000/

[Copilot]

Pull Request Overview

This PR fixes a panic in the CheckProposedVersion function when handling legacy revision cases where there's no local HLV (Hybrid Logical Vector) but the parent revision doesn't match the local current revision.

Key changes:

• Added a condition to return conflict status with current revision when no HLV exists but revisions don't match
• Updated test client conflict detection logic to handle revision tree IDs and missing HLV cases
• Added a comprehensive test case to reproduce the panic scenario

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
db/crud.go	Added null HLV check in CheckProposedVersion to prevent panic and return proper conflict status
rest/utilities_testing_blip_client.go	Updated conflict detection method to accept revOptions and handle revision tree ID cases
rest/blip_api_crud_test.go	Added test case reproducing the panic scenario in legacy revision handling