fix: refresh loads in async client

[mikemikimike]

Closes #3772

AsyncMilvusClient.refresh_load() currently delegates to the low-level progress query instead of requesting a load refresh. As a result, async callers and optimize_collection() can return without reloading refreshed segments.

This change reuses the existing async load paths: collections call load_collection(..., _refresh=True), while partition refreshes call load_partitions(..., _refresh=True). Regression tests verify both routes and ensure the progress-only handler is not used.

Validation: async client unit tests passed (81); Ruff format/check and git diff --check passed. A targeted mypy run is blocked by existing missing grpc/milvus-lite stubs and generated protobuf stub syntax errors.

This contribution was prepared with assistance from an AI agent.

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[linhongyu510]

I checked out this PR in an isolated worktree and ran the complete async client operations test module with the PR source forced on PYTHONPATH:

pytest tests/unit/test_async_milvus_client_ops.py -q
81 passed, 3 warnings

Both new regression cases pass:

• collection refresh delegates to load_collection(..., _refresh=True);
• partition refresh delegates to load_partitions(..., _refresh=True).

I also verified that the progress-only handler is not used by either path. I did not find a correctness issue in the changed scope.