add unit test coverage for WorkerBatchInternalQueue

[aglinxinyuan]

Background

WorkerBatchInternalQueue in engine/architecture/pythonworker currently lacks a dedicated unit-spec. It is a trait providing the per-DP-thread mailbox used by the Python worker, with two-priority queues (data + control) and per-channel in-flight byte accounting via getQueuedCredit.

Behavior to pin

Surface	Contract
enqueueData + getElement	round-trips a DataElement
enqueueCommand + getElement	round-trips a ControlElement
enqueueActorCommand + getElement	round-trips an ActorCommandElement
Multi-priority dispatch	control elements are returned before data elements when both are queued (queue priorities 0 < 1)
getDataQueueLength	reports the count of data-queue items (excludes control)
getControlQueueLength / isControlQueueEmpty	reports the count of control-queue items
disableDataQueue	stops getElement from picking from the data queue (control items still flow)
enableDataQueue	re-enables the data queue
getQueuedCredit(sender)	equals 0 when nothing has been enqueued
getQueuedCredit after enqueueData(DataFrame)	tracks bytes IN minus bytes OUT for that sender
getQueuedCredit after getElement consumes the data	returns to 0 (in == out)
getQueuedCredit for non-DataFrame payloads	does not increment in/out byte counters
getQueuedCredit across distinct senders	keeps each sender's accounting independent

Scope

• New spec file: WorkerBatchInternalQueueSpec.scala (matches the <srcClassName>Spec.scala convention).
• The trait is instantiated through a small test-only subclass.
• No production-code changes.