Add Milvus memory backend

[zc277584121]

Summary

• Add an optional Milvus memory backend backed by pymilvus MilvusClient, with Milvus Lite as the default local URI and support for server or managed endpoints through uri/token options.
• Extend memory provider config and credential splitting so Milvus tokens stay out of config.json.
• Add Milvus Lite coverage for write, read, list, tree, search, persistence, and dimension mismatch behavior.
• Document install and basic usage.

Verification

• uv run ruff check src tests example
• uv run ruff format --check src tests example
• uv run mypy src
• Full pytest suite with network-dependent provider tests deselected: 893 passed, 53 skipped, 30 deselected.
• uv run --extra milvus pytest tests/memory/milvus_backend: 5 passed, 1 skipped.

No breaking changes.

[zc277584121]

Hi @Tokisakix @kangkangzi2025, this optional Milvus memory backend PR is ready for review when you have a chance.

It keeps the local backend as the default and adds Milvus as an optional backend with Milvus Lite, Milvus server, and Zilliz Cloud configuration support, plus conformance coverage. Happy to adapt it to the preferred memory backend conventions.