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/transfer.rs

@@ -473,9 +473,9 @@ pub async fn execute_on_pool(state: &AppState, pool_key: &str, sql: &str) -> Res
     let pool = connections.get(pool_key).ok_or("Connection not found")?;
 
     match pool {
-        PoolKind::Mysql(p, bare) => {
+        PoolKind::Mysql(p, mode) => {
             let p = p.clone();
-            let bare = *bare;
+            let bare = *mode == crate::connection::MysqlMode::Bare;
             drop(connections);
             db::mysql::execute_query(&p, sql, bare).await
         }