[4.1.2 Backport] CBG-5591: stop indexes being built on default when not required

[bbrks]

CBG-5591

Unclean cherry pick of #8495 to 4.1.2
Changes from main commit:

• rest/manualbucketpooltest/database_init_manager_test.go — package doesn't exist on 4.1.2; its TestDatabaseInitConcurrentDatabasesSameBucket lives in rest/database_init_manager_test.go, so the InitializeDatabase signature change was applied there instead
• rest/database_init_manager_test.go — dbConfig.setup() takes an extra forcePerBucketAuth arg on 4.1.2, so the ported tests use the 6-arg form

Stacked on #8522 (CBG-5414 backport), which this depends on - with that in place, rest/database_init_manager.go and TestBuildCollectionIndexData match main exactly.

[Copilot]

Pull request overview

Backports the main-branch fix for CBG-5591 to 4.1.2, refining database index initialization so Sync Gateway doesn’t (re)build metadata indexes on _default._default when they’re no longer required—especially after system-metadata migration has completed—avoiding wasted work and initialization failures when _default has been dropped.

Changes:

• Plumbs a migrationComplete signal into DatabaseInitManager so index initialization can exclude vestigial _default metadata indexes post-migration.
• Updates collection-to-index-set planning (buildCollectionIndexData) to treat _default’s “data role” and “metadata role” independently.
• Reduces test bucket pool usage in ISGR test setup by only acquiring buckets when configs don’t already provide one, and adds/updates tests to cover the new index-init behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
rest/utilities_testing_isgr.go	Avoids double-reserving bucket-pool buckets by only allocating a test bucket when the caller didn’t supply one.
rest/server_context.go	Detects migration completion (via dual metadata store state) and passes it through to index initialization.
rest/indextest/index_test.go	Updates async-init tests’ expected collection counts for the new index-init behavior.
rest/database_init_manager.go	Extends initialization APIs to accept migrationComplete and updates index planning logic to exclude _default metadata indexes post-migration.
rest/database_init_manager_test.go	Adds/updates tests to validate correct collection/index initialization across legacy vs system-metadata modes and migration states.
rest/admin_api.go	Ensures manual index-init requests pass migration completion state to the init manager.

Suppressed comments (1)

rest/indextest/index_test.go:505

• Same as above: without explicitly setting UseSystemMobileMetadataCollection, this test no longer guarantees it is exercising the system-metadata + migration-complete path (and the updated expected collection count can be satisfied by legacy mode too).

	dbConfig := makeDbConfig(t, tb, syncFunc, importFilter)
	dbConfig.StartOffline = base.Ptr(true)
	dbConfigPayload, err := json.Marshal(dbConfig)