chip ingress smoke test: verify PublishBatch via dedicated callback

[pkcll]

Summary

Add PublishBatchFunc to the CHiP test sink so the smoke test can directly observe PublishBatch RPCs without relying on grpc.Method(ctx) inspection.

Changes

• chip-testsink/server.go — Add PublishBatchFn type and PublishBatchFunc field to Config. When set, PublishBatch delegates to it; otherwise falls back to per-event PublishFunc.
• chip_testsink_helpers.go — Add ChipTestSinkOpt functional option and WithPublishBatchFunc helper. StartChipTestSink now accepts variadic opts.
• chip_ingress_batching_test.go — Use PublishBatchFunc to count batch RPCs and total batched events directly. Removes grpc.Method string matching.

Testing

Builds cleanly via go build ./... in system-tests/tests.