bug: Transactional topic example exhausts producer_id limit with random IDs

Bug Report

YDB Python SDK version:

Environment

Environment is irrelevant for reproduction - issue is in code example

Current behavior:

In the transactional topic write example (topic_transactions_async_example.py):

1. Each call to tx_writer without explicit producer_id generates a new random producer_id
2. Transaction retries create additional producer_ids
3. This rapidly consumes the producer_id limit (100K per 14 days)
4. The example encourages production-unsafe usage patterns

Expected behavior:

Examples should demonstrate best practices:

1. Explicit producer_id should be used to avoid random ID generation
2. Example should be resilient to transaction retries
3. Should show either fixed producer_id or partition_id usage
4. Should not risk exhausting producer_id limits in production scenarios

Steps to reproduce:

1. Use example from topic_transactions_async_example.py
2. Run write_with_tx_example method in a loop or with frequent retries
3. Observe rapid growth of producer_ids in the topic

Related code:
https://github.com/ydb-platform/ydb-python-sdk/blob/main/examples/topic/topic_transactions_async_example.py#L28

async def write_with_tx_example(driver: ydb.aio.Driver, topic: str, message_count: int = 10):
    async with ydb.aio.QuerySessionPool(driver) as session_pool:

        async def callee(tx: ydb.aio.QueryTxContext):
            tx_writer: ydb.TopicTxWriterAsyncIO = driver.topic_client.tx_writer(tx, topic)

            for i in range(message_count):
                async with await tx.execute(query=f"select {i} as res;") as result_stream:
                    async for result_set in result_stream:
                        message = str(result_set.rows[0]["res"])
                        await tx_writer.write(ydb.TopicWriterMessage(message))
                        print(f"Message {result_set.rows[0]['res']} was written with tx.")

        await session_pool.retry_tx_async(callee)

Other information:

Current example can exhaust 100K producer_id limit under moderate load

Possible Solutions
Fixed producer_id
Explicit partition selection