fix(store): flush remote upserts for immediate search

[dommonkhouse]

Summary

• Flush remote Milvus collections after upsert so newly captured chunks are immediately searchable
• Keep Milvus Lite behaviour unchanged
• Add regression coverage for remote and local upsert behaviour

Verification

• uv run pytest tests/test_store.py
• uv run ruff check src/memsearch/store.py tests/test_store.py
• git diff --check

Note: full suite currently has one unrelated failure in tests/test_embeddings_openai.py::test_embed_deterministic due exact float comparison drift.

[dommonkhouse]

Fresh reproduction from a remote Milvus setup using upstream vanilla memsearch[onnx]==0.4.3:\n\n- Temporary collection: ms_vanilla_probe_<timestamp>\n- memsearch index reported Indexed 1 chunks.\n- Immediate memsearch stats for that collection reported Total indexed chunks: 0\n- Searches through the agreed 5-second window returned []\n- The same test shape using my fork with this PR's flush patch reported Total indexed chunks: 1 immediately and search found the probe at poll 0\n\nSo Milvus auto-flush is not enough for the write-then-search workflow on remote Milvus; an explicit flush after upsert is still needed for immediate visibility.

[dommonkhouse]

Ready for maintainer review and merge. I verified this PR is non-draft, mergeable, and has no configured checks reporting failures.