CBG-5255 [4.0.5 backports] allow metadata id to be nil while syncinfo exists

[gregns1]

CBG-5255

Backports #8059

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

• https://jenkins.sgwdev.com/job/SyncGatewayIntegration/0000/

[Copilot]

Pull request overview

This PR backports upstream work to handle a legacy/upgrade edge case where a _sync:syncInfo document can exist with a metadata_version but without a metadataID (e.g., after attachment migration with non-persistent config). The goal is to avoid incorrectly forcing resync/metadata-ID changes in that scenario.

Changes:

• Makes base.SyncInfo.MetadataID optional (*string) and updates metadata ID computation to tolerate a missing metadataID in _sync:syncInfo.
• Updates base.InitSyncInfo to return both requiresResync and requiresAttachmentMigration, and introduces logic that treats metadataID=nil as “no resync required”.
• Expands test coverage, including new error-path testing via a new LeakyDataStore.AddCallback, and stabilizes upgrade tests by waiting for attachment migration completion.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
rest/upgradetest/upgrade_registry_test.go	Waits for attachment migration completion before upgrading, and improves failure diagnostics when DB doesn’t come online.
rest/config_manager.go	Handles syncInfo.MetadataID == nil safely when computing metadata IDs.
rest/config_manager_test.go	Updates test syncInfo payload to explicitly set metadataID for the “associated with a different DB” case.
db/database_test.go	Updates SyncInfo assertions for pointer metadataID and validates the new requiresResync return from InitSyncInfo.
base/leaky_datastore.go	Adds AddCallback hook to simulate Add failures/CAS races in tests.
base/leaky_bucket.go	Extends LeakyBucketConfig with the new AddCallback.
base/constants_syncdocs.go	Makes SyncInfo.MetadataID optional, adds requiresResync helper, and centralizes the minimum attachment-migration metadata version constant.
base/constants_syncdocs_test.go	Adds coverage for InitSyncInfo error paths and CAS-mismatch scenarios using AddCallback.