[Dataflow Streaming] Enable state tag encoding v2 based on backend flag

[arunpandianp]

No description provided.

[arunpandianp]

@scwhittle

[gemini-code-assist]

Summary of Changes

Hello @arunpandianp, 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 introduces a mechanism to enable State Tag Encoding V2 in Dataflow streaming jobs, controlled by a configuration flag from the Dataflow backend. This change centralizes the control over which encoding version is used, allowing for a more robust and managed rollout of this feature. The worker's execution context now dynamically determines the appropriate encoding at runtime, replacing a previous experiment-based activation.

Highlights

• Dynamic State Tag Encoding: The Dataflow worker can now dynamically select between WindmillTagEncodingV1 and WindmillTagEncodingV2 based on a configuration flag received from the Dataflow backend.
• Backend-Driven Configuration: The decision to use State Tag Encoding V2 is now controlled by the StreamingEngineStateTagEncodingVersion field provided by the Dataflow backend, moving away from a local experiment flag.
• Refactored StreamingModeExecutionContext: The windmillTagEncoding field in StreamingModeExecutionContext is no longer final and its initialization has been moved to the start method, allowing for runtime configuration.
• Global Configuration Update: A new enableStateTagEncodingV2 boolean field has been added to StreamingGlobalConfig to store the backend-provided state tag encoding version.
• Experiment Flag Removal: The UNSTABLE_WINDMILL_TAG_ENCODING_EXPERIMENT and its associated logic have been removed, indicating a more stable and controlled rollout mechanism for V2 encoding.
• Dependency Update: The google_api_services_dataflow client library version has been updated to v1b3-rev20260118.
• Comprehensive Testing: New tests have been added to ensure that the WindmillTagEncoding is correctly selected based on the global configuration and that the backend flag is properly processed.

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[arunpandianp]

R: @scwhittle

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