Add support for Ephemeral TaskLists

[natemort]

Update Sessions to use Ephemeral TaskLists behind a feature flag. This ensures that the per-host TaskList is automatically removed once it is no longer used.

This should only be enabled once the server fully supports Ephemeral TaskLists as it will otherwise return errors for the unknown TaskListKind.

What changed?

• Added support for dispatching session activities to ephemeral TaskLists and polling Ephemeral TaskLists

Why?

• To support automatically removing session and integration test TaskLists when they're no longer used.

How did you test it?

• Unit/integration test. Will run through canary tests before enabling.

Potential risks

• Errors in task polling would significantly impact users, so this is a high risk change.

[codecov]

Codecov Report

❌ Patch coverage is 96.07843% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.72%. Comparing base (97eba63) to head (33955d2).

Files with missing lines	Patch %	Lines
internal/internal_event_handlers.go	50.00%	2 Missing ⚠️

Files with missing lines	Coverage Δ
internal/activity_task_handler.go	96.77% <100.00%> (ø)
internal/client.go	76.06% <100.00%> (+0.20%)	⬆️
internal/internal_activity.go	62.15% <ø> (ø)
internal/internal_task_handlers.go	81.90% <100.00%> (+0.03%)	⬆️
internal/internal_task_pollers.go	82.85% <100.00%> (-0.62%)	⬇️
internal/internal_utils.go	76.58% <ø> (ø)
internal/internal_worker.go	82.59% <100.00%> (+5.98%)	⬆️
internal/internal_worker_base.go	73.56% <ø> (+1.32%)	⬆️
internal/workflow.go	76.58% <100.00%> (+0.14%)	⬆️
internal/workflow_replayer.go	77.82% <100.00%> (+0.30%)	⬆️
... and 2 more

... and 1 file with indirect coverage changes

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