Backlog statistics: Handle "unversioned to ramping" case

[Shivs11]

What changed?

• WISOTT
• To be a bit more descriptive: This PR attributes a portion of the unversioned backlog when a current/ramping version is set. However, I had forgotten to properly consider the case when someone ramps to unversioned.

Why?

• Correctness.

How did you test it?

• built
• run locally and tested manually
• covered by existing tests
• added new unit test(s)
• added new functional test(s)

Potential risks

• None

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Note

Ensures task queue stats correctly handle ramping to the unversioned target.

• Adjusts Describe attribution in task_queue_partition_manager.go to use isRamping from CalculateTaskQueueVersioningInfo and avoid subtracting ramp share from unversioned when ramping target is unversioned; current share still absorbed appropriately
• Adds functional tests validating current absorbs (100 - ramp%) of unversioned backlog when ramping to unversioned (single/multi-partition) and keeps legacy stats reporting total backlog
• Adds unit test in worker_versioning_test.go for mixed new-format data where current is versioned and ramping is unversioned
• Minor test naming/label adjustments

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