fix(sdk): stream should terminate after consumer shutdown

[haubur]

Which issue does this PR address?

Closes #3910

Rationale

After consumer.shutdown() calling consumer.next() was still possible. Causing

• for a standalone consumer to poll fresh from the server, while storing offsets was unreachable due to dropping the channel on shutdown()
• for a consumer group to hang indefinitely while trying to build a new poll future

What changed?

When writing the issue I said that the preferred way would be to return an IggyError to allow user handling.
However, calling next() on a shut down consumer now returns: Poll::Ready(None) indicating a terminated stream as intended by the futures crate: https://docs.rs/futures/latest/futures/prelude/trait.Stream.html#:~:text=Poll%3A%3AReady%28None%29%20means%20that%20the%20stream%20has%20terminated%2C%20and%20poll%5Fnext%20should%20not%20be%20invoked%20again

Local Execution

• Passed
• Pre-commit hooks ran

[github-actions]

Thanks for the PR. It is labeled S-waiting-on-review and queued for review.

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See CONTRIBUTING.md for details.

[codecov]

Codecov Report

❌ Patch coverage is 93.10345% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.85%. Comparing base (1a98748) to head (8a1ee5f).

Files with missing lines	Patch %	Lines
core/sdk/src/clients/consumer.rs	93.10%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3964       +/-   ##
=============================================
- Coverage     83.99%   69.85%   -14.14%     
  Complexity     1358     1358               
=============================================
  Files          1215     1215               
  Lines        170548   146083    -24465     
  Branches     138292   113829    -24463     
=============================================
- Hits         143252   102049    -41203     
- Misses        23439    40131    +16692     
- Partials       3857     3903       +46     

Components	Coverage Δ
Rust Core	66.60% <93.10%> (-18.29%)	⬇️
Java SDK	66.67% <ø> (ø)
C# SDK	74.96% <ø> (ø)
Python SDK	90.13% <ø> (ø)
PHP SDK	84.48% <ø> (ø)
Node SDK	95.90% <ø> (+0.09%)	⬆️
Go SDK	68.29% <ø> (ø)

Files with missing lines	Coverage Δ
core/sdk/src/clients/consumer.rs	77.05% <93.10%> (+5.27%)	⬆️

... and 317 files with indirect coverage changes

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