Fix blocksToDelete shipper initialization race

[Rohithmatham12]

What this changes

• Initializes the per-tenant Thanos shipper before assigning userDB.db in createTSDB.
• Keeps blocksToDelete from observing a partially initialized userTSDB where db is visible but shipper is still being written.
• Leaves the shipped-block cache initialization after the TSDB is fully assigned, preserving the existing behavior.

Why
tsdb.Open installs userDB.blocksToDelete as a callback and can run block reload work concurrently. Previously createTSDB assigned userDB.db before creating userDB.shipper, so the callback could read userDB.shipper while the constructor wrote it. Creating the shipper before making db visible removes that race window without adding locks to the callback path.

Fixes #6413.

Testing

• git diff --check
• GOCACHE=/private/tmp/cortex-go-cache-6413 GOMODCACHE=/private/tmp/cortex-go-mod-cache go test -tags "netgo slicelabels" ./pkg/ingester -run "TestIngester_shipBlocks|TestIngester_dontShipBlocksWhenTenantDeletionMarkerIsPresent|TestIngester_TSDB"
• GOCACHE=/private/tmp/cortex-go-cache-6413-full GOMODCACHE=/private/tmp/cortex-go-mod-cache go test -tags "netgo slicelabels" ./pkg/ingester