## What changed?
<!-- Describe what has changed in this PR -->
Drop speculative WFT even if it had events.

## Why?
<!-- Tell your future self why have you made these changes -->
Special compatibility flag was
[added](temporalio/api#467) to
`RespondWorkflowTaskCompletedRequest`. If this flag is set to true, then
server can drop speculative WFT with update rejections, even if it
shipped events to the worker. SDK support will be added later.

## How did you test it?
<!-- How have you verified this change? Tested locally? Added a unit
test? Checked in staging env? -->
Added new unit tests.

## Potential risks
<!-- Assuming the worst case, what can be broken when deploying this
change to production? -->
No risks. Compatibility flag is used to support backward compatibility
and don't drop speculative WFT if worker doesn't support it (old go
SDKs).

## Documentation
<!-- Have you made sure this change doesn't falsify anything currently
stated in `docs/`? If significant
new behavior is added, have you described that in `docs/`? -->
Yes, updated.

## Is hotfix candidate?
<!-- Is this PR a hotfix candidate or does it require a notification to
be sent to the broader community? (Yes/No) -->
No.