[Bug] Fix debezium-json cannot infer primary keys from standard Debezium message key

[zhang-arvin]

Purpose

• Fixes [Bug] Kafka debezium-json cannot infer primary keys from standard Debezium message key #8961
• Add test case for value-with-schema-envelope scenario where standard Debezium JSON records have schema+payload envelope in the value
• The primary key extraction from Kafka message key was already implemented in PR [flink][cdc] Fix missing primary keys for Debezium records #9465
• This PR adds a test covering the exact scenario from the issue

Tests

• Added testPrimaryKeysFromValueWithSchemaEnvelope test
• All existing tests continue to pass

[JingsongLi]

+1