[jasperblues]

The chat store could not run on Postgres, and its Oracle tests ran only with ORACLE_TESTS=true.

Before

• Drivine 0.1.9: Neo4j, FalkorDB, Memgraph, Oracle.
• Oracle tests skipped in a plain mvn test and in CI.

Now

• Drivine 0.2.0: Postgres 14+ as a fifth database, plain SQL with pgvector for the message vector index. No main-code change.
• The repository and session-kind suites run on Postgres (pgvector/pgvector:pg17 container) and on Oracle Free in every mvn test, so in CI too.
• README: Postgres row and ### Postgres section; Oracle no longer described as opt-in.

Key change

• ChatSessionRepositoryOracleTest's body moves to ChatSessionRepositorySqlTests; ChatSessionRepositoryOracleTest and ChatSessionRepositoryPostgresTest subclass it, as SessionKindsAndParts{Oracle,Postgres}Test subclass SessionKindsAndPartsTests.
• application-postgres.yml sets type: POSTGRES; org.postgresql:postgresql added as a test dependency.
• OracleCypherFixtures (deprecated in 0.2.0) replaced by SqlCypherFixtures.