Add optional Milvus vector store

[zc277584121]

Summary

• add an optional MilvusVectorStore backed by pymilvus.MilvusClient
• support a Milvus Lite file by default and remote Milvus or Zilliz Cloud through URI and token configuration
• preserve the existing vector-store contract for payload reconstruction, hashes, clear and copy behavior, COSINE scores, async calls, and inherited MMR
• add explicit schema validation with a COSINE AUTOINDEX and a compatibility guard for the Milvus Lite 3.0 score regression
• add the milvus optional dependency, lockfile entries, shared provider tests, and README usage examples

Compatibility

The default NumpyVectorStore behavior is unchanged. Importing PaperQA without the optional dependency continues to work, while constructing MilvusVectorStore without it raises an installation-focused error.

Testing

• provider and Milvus Lite tests on Python 3.11: 8 passed
• provider and Milvus Lite tests on Python 3.13: 8 passed
• live embedding and Milvus Lite retrieval smoke test
• changed-file pre-commit hooks
• Ruff, Black, mypy, pylint, and refurb checks
• package build and lockfile validation

[Copilot]

Pull request overview

This PR adds a new optional Milvus-backed vector store implementation to PaperQA while keeping the default in-memory vector store behavior unchanged and preserving the existing vector-store contract.

Changes:

• Introduces MilvusVectorStore implemented on top of pymilvus.MilvusClient, including schema/index validation and a Milvus Lite 3.0 score-normalization guard.
• Adds shared vector-store conformance tests plus Milvus-specific tests (schema, copy semantics, dependency error path, and Docs integration).
• Wires up the optional milvus extra (plus lockfile updates) and documents usage in the README.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
uv.lock	Adds Milvus-related dependencies (and lockfile refresh) to support the new optional extra.
tests/test_vector_stores.py	Adds provider-style conformance coverage across Numpy/Qdrant/Milvus plus Milvus-specific behavior tests.
src/paperqa/llms.py	Implements MilvusVectorStore, including collection/schema/index management and search behavior.
src/paperqa/__init__.py	Exposes MilvusVectorStore in the package public API.
README.md	Documents how to install and use Milvus (Lite and remote) as a vector store.
pyproject.toml	Adds the milvus optional dependency extra and includes it in the dev extra set.

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