fix(async): preserve optimization after result wait timeout

[Kuang-xianxin]

Problem

AsyncOptimizeTask.result(timeout=...) passes its owned background task directly to asyncio.wait_for. A short wait cancels the optimization, affects other result waiters, and prevents a later retry. For the preview optimize(wait=False) API this can stop the client workflow during compaction before index rebuild and load refresh.

Cancellation also needs to distinguish ownership: a caller holding an AsyncOptimizeTask can wait again or explicitly cancel it, while optimize(wait=True) does not return a handle when it exits early.

Change

• Shield the background task in both timed and untimed result() waits. A local wait timeout raises the existing timeout error; caller cancellation propagates as CancelledError, allowing standard asyncio.wait_for and asyncio.timeout wrappers to work without cancelling the shared optimization.
• Keep explicit task.cancel() behavior and the optimization's own timeout forwarding.
• In optimize(wait=True), cancel and await unfinished owned work before returning a wait timeout or propagating caller cancellation. This prevents detaching an inaccessible task and retrieves its cancellation exception.
• Document the ownership distinction. Server-side compaction cancellation behavior is unchanged.

Regression coverage includes later success/failure after a short wait, concurrent waiters, explicit task cancellation, external timeout wrappers, and zero/positive timeouts or cancellation of optimize(wait=True). A public optimize(wait=False) workflow test uses mocked RPC responses and verifies load refresh after a timed-out result wait.

Validation

Python 3.13.14 on Windows:

• Original source: 5 initial regression cases failed.
• Before the review revision: 9 additional cancellation/ownership regressions failed on d22d7af.
• Latest revision: 148 passed with:

uv run --no-sync pytest tests/unit/test_async_optimize_task.py tests/unit/test_async_milvus_client_ops.py tests/unit/test_optimize_task.py -q -o log_cli=false --tb=short

• Ruff and Black checks pass on all four changed files.
• The asyncio.timeout context test is skipped on Python versions before 3.11; wait_for cases cover older versions.
• No live Milvus server or performance benchmark was used; this is a client task-lifecycle fix.

Found through source inspection. Searches did not find a matching optimization/result-timeout fix; the existing async refresh-load PRs concern separate RPC options.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.24%. Comparing base (a4b3b38) to head (d22d7af).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3783      +/-   ##
==========================================
+ Coverage   94.22%   94.24%   +0.01%     
==========================================
  Files          77       77              
  Lines       16204    16204              
==========================================
+ Hits        15268    15271       +3     
+ Misses        936      933       -3     

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