Ensure that the payload is consumed on Elements.Data and Elements.Timers when is_last = true

BeamFnDataInboundObserver drops the bytes instead of passing it forward on the last message: https://github.com/apache/beam/blob/cd4b7f3b1af4f51bdab1a0b1a98f94b5288c09ec/sdks/java/fn-execution/src/main/java/org/apache/beam/sdk/fn/data/BeamFnDataInboundObserver.java#L58

data_plane.py drops the timers payload but seems to do the right thing for data:
https://github.com/apache/beam/blob/cd4b7f3b1af4f51bdab1a0b1a98f94b5288c09ec/sdks/python/apache_beam/runners/worker/data_plane.py#L343

data_mgr.go drops the data payload (timers don't seem to be supported):
https://github.com/apache/beam/blob/cd4b7f3b1af4f51bdab1a0b1a98f94b5288c09ec/sdks/go/pkg/beam/core/runtime/harness/datamgr.go#L297

Imported from Jira [BEAM-13142](https://issues.apache.org/jira/browse/BEAM-13142). Original Jira may contain additional context.
Reported by: lcwik.

Start with BeamFnDataInboundObserver.java, data_plane.py, and datamgr.go at the referenced locations, comparing how final Elements.Data and Elements.Timers messages are handled. Confirm that payload bytes are forwarded when is_last=true in each supported path, then verify the relevant runner harness behavior with existing tests.

Written by the indexing model from the issue text.