feat: add synchronous LoadCollection warmup option

[sunby]

Summary

Add explicit warmup="sync" handling to the shared LoadCollection request builder used by synchronous and asynchronous handlers.

• Encode the option as LoadCollectionRequest.load_params["warmup"] = "sync".
• Preserve existing behavior when the option is omitted.
• Reject unsupported values and explicit sync together with refresh / _refresh.
• Preserve existing load priority, replica and field-selection options.

This is a clean, single-commit PR based on upstream master (a4b3b38b): only pymilvus/client/prepare.py and tests/unit/test_load_warmup.py change, with 37 added lines. The fork's namespace commits and local protobuf submodule changes are not included.

References

• Related issue: [Enhancement]: support user specified warmup milvus#47371 (fine-grained warmup controls; this adds a separate load-time override).
• Server implementation, still under development in a fork: feat: add gated synchronous LoadCollection warmup sunby/milvus#17
• Design: https://github.com/sunby/milvus/blob/b2b2c343926f37e3067f547889b31f30a04216fb/docs/design-docs/design_docs/20260907-load-collection-force-sync-warmup.md
• Supersedes the SDK fork PR: feat: add synchronous LoadCollection warmup option sunby/pymilvus#1

Validation

Verified in an isolated checkout of this upstream-based commit:

• pytest tests/unit/prepare/test_collection.py tests/unit/test_load_warmup.py -q: 73 passed, including all 10 new warmup cases.
• Ruff check and Ruff format check passed for both changed files.
• Black format check passed for both changed files.
• git diff --check passed.

Release boundary

Keep this PR as Draft pending the corresponding server support and SDK/server E2E acceptance. Request serialization and validation are tested; actual synchronous cache warmup is not verified by these tests. The proposed server feature gate remains disabled by default. This SDK change does not negotiate capabilities and cannot guarantee warmup on older or otherwise unsupported servers.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.22%. Comparing base (a4b3b38) to head (f34ab6a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3786   +/-   ##
=======================================
  Coverage   94.22%   94.22%           
=======================================
  Files          77       77           
  Lines       16204    16210    +6     
=======================================
+ Hits        15268    15274    +6     
  Misses        936      936           

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