REST ConfigResponse ignores server overrides

[shyjsarah]

Problem

The REST config response currently deserializes only defaults. If a REST server returns an overrides map, serde ignores it and client-provided options incorrectly take precedence.

For example, given:

{
  "defaults": {"data-token.enabled": "false"},
  "overrides": {"data-token.enabled": "true"}
}

a client option of data-token.enabled=false remains false. This differs from the Java Paimon REST client semantics and can disable server-required behavior.

The Java client also accepts missing or null config maps and filters null values after merging. In particular, a null value in overrides removes a value supplied by defaults or the client.

Expected behavior

Match the Java REST client semantics:

defaults < client options < overrides

• Missing or null defaults / overrides maps behave as empty maps.
• Entries whose final merged value is null are removed.
• Responses without an overrides field remain backward compatible.