feat: auto-detect OceanBase Oracle mode and use Oracle-style schema queries (#155)

Introduce MysqlMode enum to distinguish Normal/Bare/OceanBaseOracle MySQL
connections. On connect, if driver_profile contains "oceanbase", detect
tenant mode via SHOW VARIABLES LIKE 'ob_compatibility_mode'. When Oracle
mode is detected, schema queries use ALL_TABLES/ALL_TAB_COLUMNS/etc.

crates/dbx-core/src/db/mod.rs

@@ -6,6 +6,7 @@ pub mod file_validator;
 pub mod gaussdb_driver;
 pub mod mongo_driver;
 pub mod mysql;
+pub mod ob_oracle;
 pub mod oracle_driver;
 pub mod postgres;
 pub mod redis_driver;