Sharded DCP: use common code

[torcolvin]

Creates common code for sharded DCP to share between import and resync.

• Create CollectionNames struct for common use. This type already exists in sgbucket.FeedArguments and probably could be pushed to sg-bucket in the future. It would probably be better to have a type that is map[string]map[string]{}struct in the future.
• Add some error checking for ResyncManger options structs.
• move ctx = CorrelationIDLogCtx(ctx, DCPImportFeedID) back to import listener in preparation for resync usage

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

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGatewayIntegration/0000/

[Copilot]

Pull request overview

Refactors sharded DCP-related code to share common collection-name handling between import and resync, and tightens some resync manager option validation.

Changes:

• Introduces base.CollectionNames (+ helpers) and migrates resync request/test usage to it.
• Adds resync manager option type-checking to avoid panics on invalid options.
• Moves import DCP correlation logging context setup back into the import listener and centralizes some DB context helpers (scope name, sharded DCP capability, collection name aggregation).

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
rest/indextest/index_test.go	Updates resync payload construction to use base.NewCollectionNames.
rest/api.go	Changes resync POST request body type for scopes to base.CollectionNames.
rest/adminapitest/collections_admin_api_test.go	Updates admin API resync tests to use base.NewCollectionNames.
db/import_listener.go	Refactors import feed startup to use shared helpers and sharded DCP decision logic.
db/database.go	Adds scopeName, useShardedDCP(), and collectionNames() helpers; enforces single-scope DB config.
db/background_mgr_resync_dcp_test.go	Updates resync DCP manager tests to pass base.CollectionNames.
db/background_mgr_resync_dcp.go	Switches resync collection tracking to base.CollectionNames and adds option validation.
base/dcp_sharded.go	Removes correlation ID context assignment from sharded DCP startup; updates comment wording.
base/collection_common.go	Adds CollectionNames type and helper methods for common scope/collection mapping.