[Dataflow Streaming] Enforce that get data requests for the same work item are not batched.

[scwhittle]

Such batches are rejected by the backend so we should prevent them to avoid stuckness. Parallel requests for the same work item are unexpected but could be caused by bugs in the harness or by incorrect parallel state fetching from a single bundle.

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[scwhittle]

R: @arunpandianp

[gemini-code-assist]

Summary of Changes

Hello @scwhittle, 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 addresses a critical issue in the Dataflow Streaming worker where multiple 'get data' requests pertaining to the same work item could be erroneously batched together. Such batches are rejected by the backend, leading to job stuckness. The changes introduce robust validation within the batching mechanism to prevent these duplicate work items from being grouped, thereby enhancing the stability and reliability of the Dataflow worker's interaction with the Windmill service.

Highlights

• Prevented Duplicate Work Item Batching: Implemented logic to ensure that 'get data' requests for the same work item are not batched together, resolving a known issue where the backend would reject such batches and cause job stuckness.
• Refactored Request Handling: Reworked the internal representation of QueuedRequest and QueuedBatch to explicitly manage different types of data requests and track work tokens for robust batch validation.
• Improved Batching Logic: Introduced a tryAddRequest method in QueuedBatch that enforces limits on batch size and count, and critically, prevents the inclusion of duplicate work items within a single batch.
• Added Concurrency Test: Included a new test case to specifically verify that multiple concurrent requests for the same work item are correctly handled as separate requests, rather than being erroneously batched.

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[arunpandianp]

2 minor comments. LGTM otherwise. Can be merged after addressing them.