[vmagent] Cut over remote_write to new DB and verify live ingestion

[letwir]

Final completion gate.

Depends on all VictoriaMetrics imports and destination validation.

Tasks:

• Temporarily dual-write to the old and new VictoriaMetrics endpoints if supported by the deployed configuration.
• Verify the new DB receives samples from vmagent.
• Remove the old endpoint and restart Database_vmagent.
• Confirm the service is stable and sample counters/new timestamps continue increasing.

Overall work is complete only when the new DB is ingesting vmagent data.