feat: add synchronous LoadCollection warmup option

[sunby]

Summary

Encode explicit warmup="sync" in LoadCollectionRequest.load_params.

• Leave load_params unchanged when the option is omitted.
• Reject unsupported values and explicit sync together with refresh / _refresh.
• Preserve the existing namespace, load-priority and other load options.

References and scope

• Server PR: feat: add gated synchronous LoadCollection warmup milvus#17
• Related issue: [Enhancement]: support user specified warmup milvus-io/milvus#47371 (fine-grained warmup controls; this is a separate load-time override).
• Design: https://github.com/sunby/milvus/blob/0db748420fc290789eedebea72766dc5971694cd/docs/design-docs/design_docs/20260907-load-collection-force-sync-warmup.md
• Base: sunby/pymilvus:fix-namespace-requests; only pymilvus/client/prepare.py and tests/unit/test_load_warmup.py are included. The local protobuf submodule change is excluded.

Verification

• PYTHONPATH=. .venv/bin/python -m pytest tests/unit/test_load_warmup.py -q: 10 passed.
• Ruff check and format check passed for both changed files.
• git diff --check passed.

Draft to accompany the server feature: request serialization is verified, but actual synchronous warmup and SDK/server E2E are pending. The server feature gate remains disabled by default; this SDK option alone is not evidence that a deployed server supports the requested behavior.

[sunby]

Superseded by milvus-io#3786, targeting the official repository. The replacement is based on upstream master and contains only the warmup request-builder change and tests, excluding the fork namespace commits. The original branch is retained.