Preserve Oracle vector store metadata

[HuitaePark]

Fixes #3838

Problem

OracleVectorStore does not preserve user-defined metadata when a document returned by similaritySearch() is added to the vector store again.

When metadata is initially added, values such as strings are represented using standard Java types:

Map.of("conversationId", "conversation-123")

After the document is stored in Oracle and retrieved through similaritySearch(), metadata values read from the Oracle JSON column are represented as OracleJsonValue implementations.

For example, a string metadata value is returned as an Oracle JSON string value rather than a standard Java String.

When the retrieved Document is added again, OracleVectorStore.toJson() only serializes a limited set of Java scalar types:

• String
• Integer
• Float
• Double
• Boolean

Because OracleJsonValue was not handled, those metadata entries were silently skipped during serialization.

As a result, the following round trip removed the original user-defined metadata:

Add document
→ Retrieve document from Oracle
→ Add the retrieved document again
→ Retrieve it again

Before this change, the second retrieval contained only generated metadata such as:

{"distance"=0.0}

The original conversationId entry was no longer present.

Changes

This change adds support for serializing OracleJsonValue instances directly through OracleJsonGenerator.

else if (o instanceof OracleJsonValue oracleJsonValue) {
    gen.write(key, oracleJsonValue);
}

This allows metadata returned by Oracle to be written back without being silently discarded.

Testing

Added an integration test that:

1. Stores a document with user-defined metadata.
2. Retrieves the document through similaritySearch().
3. Verifies that the metadata was returned as an OracleJsonValue.
4. Adds the retrieved document to OracleVectorStore again.
5. Retrieves the document a second time.
6. Verifies that the original metadata is still present.

The regression test fails before the fix because the metadata is removed, and passes after OracleJsonValue serialization is supported.