feat(DocumentMetadata): Add payload support for Document metadata

[wuji1428]

Description

This PR enhances payload support for DocumentMetadata, allowing the storage of custom business-specific metadata fields.

1. DocumentMetadata Class Enhancements

• Added payload field: A Map<String, Object> to store custom metadata
• New constructor: DocumentMetadata(ContentBlock, String, String, Map<String, Object>) with payload parameter
• Payload access methods:
  • getPayload(): Returns unmodifiable map of all payload fields
  • getPayloadValue(String key): Retrieves individual payload value by key
  • hasPayloadKey(String key): Checks if a payload key exists
• Builder pattern enhancement:
  • payload(Map<String, Object>): Sets entire payload map
  • addPayload(String key, Object value): Adds individual payload fields

2. Document Class Enhancements

• Added convenience methods that delegate to metadata:
  • getPayload(): Access payload through Document
  • getPayloadValue(String key): Get individual payload values
  • getPayloadValueAs(String key, Class<T> targetClass): Type-safe payload retrieval with automatic conversion
  • hasPayloadKey(String key): Check payload key existence

3. Store Enhancements

Support for payload storage: All stores now support storing and retrieving documents with payload

• MilvusStore: Handles payload in search and upsert operations
• QdrantStore: Supports payload in vector database interactions
• InMemoryStore: Manages payload in memory-based document storage

4. DocumentConverter Enhancements

Payload building: All converters now build payload maps from source data

• BailianDocumentConverter
• HayStackDocumentConverter
• RAGFlowDocumentConverter
• DifyDocumentConverter

Testing

• Complete test coverage for document metadata
• Test coverage for payload of store class
• Test coverage for payload of document converter class

Checklist

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

• Code has been formatted with mvn spotless:apply
• All tests are passing (mvn test)
• Javadoc comments are complete and follow project conventions
• Related documentation has been updated (e.g. links, examples, etc.)
• Code is ready for review

Related Issues

#305

[codecov]

Codecov Report

❌ Patch coverage is 60.11236% with 71 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...re/rag/integration/dify/DifyDocumentConverter.java	39.62%	14 Missing and 18 partials ⚠️
.../integration/ragflow/RAGFlowDocumentConverter.java	68.75%	4 Missing and 6 partials ⚠️
...java/io/agentscope/core/rag/store/QdrantStore.java	56.52%	7 Missing and 3 partials ⚠️
...in/java/io/agentscope/core/rag/model/Document.java	27.27%	8 Missing ⚠️
...io/agentscope/core/rag/model/DocumentMetadata.java	76.66%	4 Missing and 3 partials ⚠️
...java/io/agentscope/core/rag/store/MilvusStore.java	82.35%	2 Missing and 1 partial ⚠️
...ntegration/haystack/HayStackDocumentConverter.java	75.00%	0 Missing and 1 partial ⚠️

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