add unit test coverage for NetworkOutputBuffer

[aglinxinyuan]

amber/src/main/scala/org/apache/texera/amber/engine/architecture/sendsemantics/partitioners/Partitioner.scala defines the NetworkOutputBuffer class — the per-receiver batched-tuple sender that every concrete Partitioner subclass uses to push data downstream. It has stateful buffer + flush semantics that nothing currently pins:

Path	Behavior to pin
addTuple below batchSize	Tuple appended to the internal buffer; nothing sent yet.
addTuple at the exact batchSize boundary	Auto-flush — a single DataFrame carrying every buffered tuple is sent to the receiver.
Multiple auto-flush cycles	Each batch produces exactly one DataFrame; tuples don't leak across batches; sequence numbers / receiver stay consistent.
flush() on a non-empty buffer	One DataFrame sent; buffer reset to empty.
flush() on an empty buffer	No-op (no DataFrame sent) — pin so a regression that sends an empty DataFrame per call breaks here.
sendState	Flush-bookended: any pending tuples drain first (in their own DataFrame), then a StateFrame is sent, then a trailing flush() runs (no-op since the post-state buffer is already empty). The bookend flushes ensure state ordering relative to data is preserved.
Constructor params	batchSize defaults to ApplicationConfig.defaultDataTransferBatchSize; can be overridden. to and dataOutputPort exposed as val.

The test wires a real NetworkOutputGateway with a recording handler so the assertions exercise the production codepath end-to-end (sequence number assignment, channel-id construction, payload type) rather than mocking the gateway.

Priority

P3 - Low

Task Type

• Testing / QA