feat(rag): add MongoDB vector store in the RAG module.

examples/functionality/vector_store/mongodb/README.md

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+# MongoDB Vector Store
+
+This example demonstrates how to use **MongoDBStore** for vector storage and semantic search in AgentScope using MongoDB's Vector Search capabilities.
+It includes comprehensive test scenarios covering CRUD operations, metadata filtering, document chunking, and distance metrics.
+
+### Quick Start
+
+Install agentscope first, and then the MongoDB dependency:
+
+```bash
+pip install pymongo
+```
+
+**Important:** Before running the example, you need to set the `MONGODB_HOST`
+environment variable with your MongoDB connection string:
+
+```bash
+# For local MongoDB
+export MONGODB_HOST="mongodb://localhost:27017/?directConnection=true"
+
+# For MongoDB Atlas (replace with your connection string)
+# export MONGODB_HOST=${YOUR_MONGODB_HOST}
+```
+
+Run the example script, which showcases adding, searching, and deleting in MongoDB vector store:
+
+```bash
+python main.py
+```
+
+> **Note:** The script connects to MongoDB Atlas or local MongoDB instance. Make sure you have a valid MongoDB connection string.
+
+## Prerequisites
+
+- Confirm your MongoDB instance supports Vector Search functionality
+- Valid MongoDB connection string (local or Atlas)
+
+## Usage
+
+### Initialize Store
+
+```python
+from agentscope.rag import MongoDBStore
+
+# For MongoDB Atlas
+store = MongoDBStore(
+    host="mongodb+srv://username:[email protected]/",
+    db_name="test_db",
+    collection_name="test_collection",
+    dimensions=768,              # Match your embedding model
+    distance="cosine",           # cosine, euclidean, or dotProduct
+)
+
+# For local MongoDB
+store = MongoDBStore(
+    host="mongodb://localhost:27017/?directConnection=true",
+    db_name="test_db",
+    collection_name="test_collection",
+    dimensions=768,
+    distance="cosine",
+)
+
+# No manual initialization needed - everything is automatic!
+# Database, collection, and vector search index are created automatically
+# when you first call add() or search()
+```
+
+### Add Documents
+
+```python
+from agentscope.rag import Document, DocMetadata
+from agentscope.message import TextBlock
+
+doc = Document(
+    metadata=DocMetadata(
+        content=TextBlock(type="text", text="Your document text"),
+        doc_id="doc_1",
+        chunk_id=0,
+        total_chunks=1,
+    ),
+    embedding=[0.1, 0.2, ...],  # Your embedding vector
+)
+
+await store.add([doc])
+```
+
+### Search
+
+```python
+results = await store.search(
+    query_embedding=[0.15, 0.25, ...],
+    limit=5,
+    score_threshold=0.9,                # Optional
+    filter={"doc_id": {"$regex": "prefix.*"}},  # Optional MongoDB filter
+)
+```
+
+### Delete
+
+```python
+# Delete by document IDs (no initialization needed)
+await store.delete(ids=["doc_1", "doc_2"])
+
+# Delete entire collection (use with caution)
+await store.delete_collection()
+
+# Delete entire database (use with caution)
+await store.delete_database()
+```
+
+## Distance Metrics
+
+| Metric | Description | Best For |
+|--------|-------------|----------|
+| **cosine** | Cosine similarity | Text embeddings (recommended) |
+| **euclidean** | Euclidean distance | Spatial data |
+| **dotProduct** | Inner Product | Recommendation systems |
+
+## Advanced Usage
+
+### Access Underlying Client
+
+```python
+client = store.get_client()
+# Use MongoDB client for advanced operations
+stats = await client[db_name].command("collStats", collection_name)
+```
+
+### Document Metadata
+
+- `content`: Text content (TextBlock)
+- `doc_id`: Unique document identifier
+- `chunk_id`: Chunk position (0-indexed)
+- `total_chunks`: Total chunks in document
+
+### Vector Search Index
+
+MongoDBStore automatically creates vector search indexes with the following configuration:
+
+```python
+{
+    "fields": [
+        {
+            "type": "vector",
+            "path": "vector",
+            "similarity": "cosine",  # or euclidean, dotProduct
+            "numDimensions": 768
+        }
+    ]
+}
+```
+
+## Connection Examples
+
+### MongoDB Atlas
+
+```python
+store = MongoDBStore(
+    host=${MONGO_ATLAS_CONNECTION_STRING},
+    db_name="production_db",
+    collection_name="documents",
+    dimensions=1536,
+    distance="cosine",
+)
+```
+
+### Local MongoDB
+
+#### Without Authentication
+
+```python
+store = MongoDBStore(
+    host="mongodb://localhost:27017?directConnection=true",
+    db_name="local_db",
+    collection_name="test_collection",
+    dimensions=768,
+    distance="cosine",
+)
+```
+
+#### With Authentication
+
+```python
+store = MongoDBStore(
+    host="mongodb://user:pass@localhost:27017/?directConnection=true",
+    db_name="test_db",
+    collection_name="test_collection",
+    dimensions=768,
+    distance="cosine",
+)
+```
+
+## References
+
+- [MongoDB Vector Search Documentation](https://www.mongodb.com/docs/atlas/atlas-search/vector-search/)
+- [MongoDB Atlas Documentation](https://www.mongodb.com/docs/atlas/)
+- [AgentScope RAG Tutorial](https://doc.agentscope.io/tutorial/task_rag.html)