[Testing] Match terminal checkpoint response semantics #773

Expected behavior

When an accepted checkpoint batch completes an execution, the emulator should return no checkpoint token and an empty Operations list. This should cover both execution success and execution failure.

This service behavior is documented in the PR #757 review discussion. The emulator findings below are confirmed by source inspection; no service response trace is included in this report.

Actual behavior

The shared checkpoint core can return a newly generated token and an unseen operation delta even when the accepted batch completes the execution. See checkpoint/core.py:128–171.

Both checkpoint entry points compute that result before applying completion or failure effects:

• executor.py:1020–1043.
• checkpoint/processor.py:118–139.

So terminal-response detection must account for the accepted batch, rather than rely only on execution status before those effects run.

The SDK distinguishes tokenless execution completion from token withdrawal in state.py:1065–1075. Ordinary terminal checkpoints against the emulator do not exercise the tokenless-completion branch.

Reproduction scenario

1. Start an execution with a valid invocation and checkpoint token.
2. Submit an accepted batch containing an EXECUTION update with action SUCCEED.
3. Observe that the response still contains a checkpoint token.
4. Repeat with EXECUTION/FAIL.
5. Include unseen operation changes to demonstrate that the terminal response can also return a nonempty operations list.

This scenario follows from source inspection at revision 52b63b5f; it has not been executed for this report.

Acceptance criteria

• Cover terminal success and failure through both checkpoint entry points.
• Return no checkpoint token and empty operations for terminal batches.
• Cover terminal batches with otherwise-returnable operation changes.
• Preserve accepted updates and completion/failure effects, including the result or error.
• Add SDK/emulator integration coverage that verifies terminal completion rather than PENDING or unnecessary reinvocation.
• Preserve existing behavior for non-terminal checkpoint batches.