Achieve exactly-once writes with the BigQuery Storage Write API using a committed-type stream

Steps

1. Don't rely on the default stream for exactly-once — it's documented as at-least-once; use it only when idempotency doesn't matter or is handled downstream.
2. For exactly-once, call CreateWriteStream with type COMMITTED to create your own stream instead of writing to the default stream.
3. On each AppendRows call, supply the expected next offset for that stream — the server only accepts the write if the offset matches, which is what gives exactly-once semantics within the stream.
4. Use GetWriteStream to check a stream's current state/offset if a client needs to resume after a crash rather than guessing where it left off.
5. Reserve PENDING streams for batch-style loads where rows must stay invisible until you call FinalizeWriteStream and explicitly commit — pending streams must be committed within 24 hours.
6. Only use the BUFFERED stream type (row-level commit via flush) if integrating with the Apache Beam BigQuery I/O connector — it's called out as an advanced type, not for general use.