feat(rag): add MemoryBlobStore for testing and fast local execution

[ramashishmaurya]

AgentScope Version

2.0.8

Description

Background & Purpose:
AgentScope currently provides LocalBlobStore and S3BlobStore for RAG document storage. However, during unit testing or lightweight agent executions where disk persistence is unnecessary, requiring disk I/O or an AWS/MinIO configuration slows down development and clutters the filesystem.

Changes Made:

• Implemented MemoryBlobStore in src/agentscope/app/rag/blob_store/_memory.py, providing a zero-dependency, fully in-memory implementation of the BlobStoreBase interface using io.BytesIO.
• Exposed MemoryBlobStore through the __init__.py interface.
• Added comprehensive test coverage in tests/blob_store_memory_test.py to validate exact byte equality, chunked reading, idempotent deletions, and proper missing-file exception handling.

How to test this PR:

1. Run pytest tests/blob_store_memory_test.py
2. Verify that all 5 tests pass instantly, proving that the mock stream correctly matches the async blob interface.

Checklist

Please check the following items before code is ready to be reviewed.

• An issue has been created for this PR
• I have read the CONTRIBUTING.md
• Docstrings are in Google style
• Related documentation has been updated (e.g. links, examples, etc.) in documentation repository
• Code is ready for review
• fixes feat(rag): add MemoryBlobStore for testing and fast local execution #3084