Handle unavailable ShutdownWorker during graceful poll shutdown

[noqcks]

Ticket

Fixes #1297.

Problem

When DescribeNamespace reports worker_poll_complete_on_shutdown=true, sdk-core enables graceful poll shutdown and lets in-flight long polls wait for server-side completion after ShutdownWorker. If ShutdownWorker returns UNIMPLEMENTED or UNAVAILABLE, that server-assisted path is unavailable, but the in-flight activity poll can still wait forever and poll_activity_task() never returns PollError::ShutDown.

Solution

This draft adds a deterministic activity-only regression test. On ShutdownWorker UNIMPLEMENTED/UNAVAILABLE, the worker disables the existing graceful poll capability. LongPollBuffer keeps normal graceful behavior, but after local shutdown it observes that capability transition and lets the in-flight poll return shutdown instead of waiting forever.

Design question: is observing the existing capability from LongPollBuffer acceptable, or should this be a more explicit poll-buffer shutdown signal?

Artifacts (if appropriate):

N/A.

[CLAassistant]

All committers have signed the CLA.