CBG-5463: complete migration when a pending db drops out of cluster

[gregns1]

CBG-5463

When a bucket is in pending state due to a db being not migrated yet, we should poll to ensure that this is still the correct state to be in. Meaning if you delete a non migrated db from the bucket it should not hold up bootstrap level migration.

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/876/

[Copilot]

Pull request overview

This PR ensures bucket-level bootstrap metadata migration doesn’t remain stuck in a pending state when the last non-migrated database is removed from a bucket, by re-checking pending migrations during the bootstrap config polling loop.

Changes:

• Add a periodic recheck in the bootstrap config polling loop to attempt completion of bucket-level metadata migration when conditions change (e.g., DB deletion).
• Extend the base.BootstrapConnection interface with an IsMigrationComplete fast-path helper and implement it for Couchbase/Rosmar backends.
• Add a regression test covering deletion of a non-migrated DB unblocking bucket bootstrap migration completion.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
rest/server_context.go	Adds periodic recheck logic to complete pending bucket-level metadata migrations during config polling.
rest/metadatamigrationtest/metadata_migration_test.go	Adds a test verifying DB deletion unblocks bootstrap migration completion.
base/rosmar_cluster.go	Implements IsMigrationComplete for Rosmar bootstrap connection.
base/bootstrap.go	Adds IsMigrationComplete to the bootstrap connection interface and implements it for Couchbase.

[bbrks]

A few comments - let me know if you disagree and we can discuss. Some of this is undefined behaviour (e.g. do we 'migrate' a bootstrap bucket if there are no databases - I think yes, but I could see an argument for no)