[4.1.1 Backport] CBG-5541: Explicitly ignore collection-scoped 'non-metadata' _sync docs from metadata migration

[bbrks]

CBG-5541

• Clean cherry-pick of CBG-5534: Explicitly ignore collection-scoped 'non-metadata' _sync docs from metadata migration #8409 for 4.1.1
• release/4.1.1 based on release/4.1.0.1 (containing [4.1.0.1 backport] CBG-5550: Revert "CBG-5262 fix missing changes with active_only=true and limit=" #8418)

Integration Tests

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

[Copilot]

Pull request overview

This backport fixes metadata migration in legacy default-collection deployments by explicitly classifying certain collection-scoped _sync: “data” documents (attachments, revision-body backups, local docs/checkpoints) as out-of-scope so they don’t count as “remaining” and wedge the migration loop.

Changes:

• Treat _sync:att:, _sync:att2:, _sync:rev:, _sync:rb:, and _sync:local: keys as out-of-scope during metadata migration range scans.
• Add a regression test that seeds these collection-scoped keys into the fallback datastore and asserts the migration completes while leaving them in place.

Reviewed changes

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

File	Description
db/metadata_migration.go	Classifies collection-scoped _sync: data-doc prefixes as out-of-scope so they don’t block migration completion.
db/background_mgr_metadata_migration_test.go	Adds regression coverage ensuring migration completes when fallback contains collection-scoped data docs and only migrates true metadata.