fix(server): apply thread pool limit carve-out to all of macOS

[djanand]

Which issue does this PR address?

• Closes bug(iggy-server) - panics at startup on Intel macOS: thread pool limit cfg excludes only aarch64 #3993

Rationale

The zero thread-pool limit was skipped only on macOS aarch64, so an Intel macOS build applied it and panicked on the first fs operation with "the thread pool is needed but no worker thread is running". compio's polling driver routes fs through the blocking pool on all of macOS, not just Apple Silicon, which is what the comment above the cfg already described and what the FIXME removed in 8ed41c7 said before it.

What changed?

The zero thread_pool_limit was skipped on macOS aarch64, so an Intel macOS build applied it and panicked on the first fs operation with "the thread pool is needed but no worker thread is running". The fix now covers target="macOS" regardless of architecture. The related comment in message_writer.rs is updated to match and drop a reference to a FIXME removed in 8ed41c7.

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

If AI tools were used, please answer:

1. Which tools? Claude
2. Scope of usage? To debug stacktrace and location of error
3. How did you verify the generated code works correctly? Pre-produced the panic before change, made the change, built and booted the server, confirmed GET /ping returns 200 and completed stream/topic/send/poll round trip via the cli. cargo fmt --all --check and clippy clean
4. Can you explain every line of the code if asked? Yes

[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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.89%. Comparing base (5916e6f) to head (dae651f).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3994      +/-   ##
============================================
- Coverage     84.91%   84.89%   -0.02%     
  Complexity     1405     1405              
============================================
  Files          1224     1224              
  Lines        179301   179301              
  Branches     145615   145616       +1     
============================================
- Hits         152250   152218      -32     
- Misses        23024    23031       +7     
- Partials       4027     4052      +25     

Components	Coverage Δ
Rust Core	85.77% <ø> (-0.03%)	⬇️
Java SDK	67.35% <ø> (ø)
C# SDK	75.40% <ø> (+0.01%)	⬆️
Python SDK	90.06% <ø> (ø)
PHP SDK	85.65% <ø> (ø)
Node SDK	96.13% <ø> (ø)
Go SDK	69.36% <ø> (+0.10%)	⬆️

Files with missing lines	Coverage Δ
core/partitions/src/messages_writer.rs	88.97% <ø> (ø)
core/server_common/src/executor.rs	98.64% <ø> (ø)

... and 41 files with indirect coverage changes

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[djanand]

/ready

[djanand]

@hubcio @ethanlin01x - can you please re-review and run the CI please ?