[Python][Java] Add support for record headers in WriteToKafka (Fixes #27033)

[PDGGK]

Summary

This PR adds support for writing Kafka record headers in the Python SDK by introducing a new cross-language transform.

Changes:

• Python: Add with_headers parameter to WriteToKafka that accepts beam.Row elements with key, value, and optional headers fields
• Java: Add WriteWithHeaders class that converts Row to ProducerRecord with headers support
• Java: Register new URN beam:transform:org.apache.beam:kafka_write_with_headers:v1
• Test: Add testConstructKafkaWriteWithHeaders in KafkaIOExternalTest

Input Schema

When with_headers=True, input elements must be beam.Row with:

• key: bytes (required)
• value: bytes (required)
• headers: List[Row(key=str, value=bytes)] (optional)
• topic: str (optional, per-record override)
• partition: int (optional)
• timestamp: long (optional)

Example Usage

# Write with headers
records = [
    beam.Row(
        key=b'k1', 
        value=b'v1', 
        headers=[beam.Row(key='h1', value=b'hv1')]
    )
]

p | beam.Create(records) | WriteToKafka(
    producer_config={'bootstrap.servers': 'localhost:9092'},
    topic='my-topic',
    with_headers=True
)

Test plan

• Java compilation passes
• testConstructKafkaWriteWithHeaders passes
• Code reviewed by Codex and Gemini

Related Issues

Fixes #27033

---

🤖 Generated with Claude Code

[gemini-code-assist]

Summary of Changes

Hello @PDGGK, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the Apache Beam Python SDK's Kafka integration by adding native support for Kafka record headers. This feature allows users to attach metadata to their Kafka messages, providing greater flexibility and richer context for data processing workflows. The implementation involves a new cross-language transform in Java to handle the conversion of structured Beam Row elements into Kafka ProducerRecords with headers, alongside updates to the Python WriteToKafka API and necessary validation.

Highlights

• Kafka Headers Support in Python SDK: The Python WriteToKafka transform now includes a with_headers parameter, enabling users to write Kafka records with custom headers.
• New Cross-Language Transform for Headers: A new Java class, WriteWithHeaders, has been introduced. This class acts as a cross-language transform, converting beam.Row elements (which can include key, value, and headers) into Kafka ProducerRecord objects.
• URN Registration: A new URN, beam:transform:org.apache.beam:kafka_write_with_headers:v1, has been registered to identify the new WriteWithHeaders external transform.
• Input Schema for Headers: When with_headers=True, input elements must be beam.Row objects with key (bytes), value (bytes), and an optional headers field (List[Row(key=str, value=bytes)]). Optional topic, partition, and timestamp fields are also supported for per-record overrides.
• Serializer Restriction: When using with_headers=True, only ByteArraySerializer is supported for both key and value serializers to ensure compatibility with header handling.

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[github-actions]

Assigning reviewers:

R: @tvalentyn for label python.
R: @chamikaramj for label java.
R: @Dippatel98 for label kafka.

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