[KYUUBI #7305][FOLLOWUP] Mirror JDBC standard setSchema in JdbcDialect

Per review feedback, replicate the JDBC `Connection#setSchema(String)` API on
JdbcDialect with a leading `conn` parameter. The default forwards to the
standard API; dialects whose driver does not support it can override to a
no-op. Drop the boolean return value — callers treat the call as if it always
takes effect. JdbcSessionImpl now records `effectiveDatabase` unconditionally
after the call, decoupling "backend connection switched" from "engine
metadata scope".

Author: wangzhigang1999

externals/kyuubi-jdbc-engine/src/main/scala/org/apache/kyuubi/engine/jdbc/dialect/JdbcDialect.scala

@@ -83,18 +83,15 @@ abstract class JdbcDialect extends SupportServiceLoader with Logging {
   def getSchemaHelper(): SchemaHelper
 
   /**
-   * Switch the current database on the backend connection.
-   *
-   * Called during session open when the client specified a database in the connection URL
-   * (populated by the Hive JDBC driver into `USE_DATABASE`). Dialects that support an
-   * in-session database context switch should override this and return `true` on success;
-   * the default is a no-op that returns `false`, meaning the backend has no notion of
-   * "current database" that applies to this session.
-   *
-   * The return value is used by `JdbcOperationManager` to decide whether the requested
-   * database can be treated as the effective schema filter for metadata operations.
+   * Mirrors the standard `Connection#setSchema(String)` JDBC API with a leading
+   * `conn` parameter, allowing dialects to customize the call. The default forwards
+   * to the standard API; dialects whose driver does not support it (e.g. Phoenix,
+   * Impala) should override this to a no-op. Callers should treat this as if it
+   * always takes effect.
    */
-  def setCurrentDatabase(connection: Connection, database: String): Boolean = false
+  def setSchema(conn: Connection, schema: String): Unit = {
+    conn.setSchema(schema)
+  }
 
   def cancelStatement(jdbcStatement: Statement): Unit = {
     if (jdbcStatement != null) {

externals/kyuubi-jdbc-engine/src/main/scala/org/apache/kyuubi/engine/jdbc/dialect/MySQLDialect.scala

@@ -150,15 +150,14 @@ class MySQLDialect extends JdbcDialect {
     query.toString()
   }
 
-  override def setCurrentDatabase(connection: Connection, database: String): Boolean = {
+  override def setSchema(conn: Connection, schema: String): Unit = {
     // mysql-connector-j makes setCatalog/setSchema mutually exclusive based on the
     // `databaseTerm` connection property: with the default CATALOG, setCatalog issues
     // COM_INIT_DB and setSchema is a silent no-op; with SCHEMA it is the inverse.
     // Calling both means exactly one fires regardless of how the user configured the
     // URL. StarRocksDialect / DorisDialect inherit this via MySQLDialect.
-    connection.setCatalog(database)
-    connection.setSchema(database)
-    true
+    conn.setCatalog(schema)
+    conn.setSchema(schema)
   }
 
   override def getTRowSetGenerator(): JdbcTRowSetGenerator = new MySQLTRowSetGenerator

externals/kyuubi-jdbc-engine/src/main/scala/org/apache/kyuubi/engine/jdbc/session/JdbcSessionImpl.scala

@@ -48,9 +48,10 @@ class JdbcSessionImpl(
   private var databaseMetaData: DatabaseMetaData = _
 
   /**
-   * The database that was successfully applied to the backend connection during session open.
-   * `None` when no `USE_DATABASE` was requested, or when the request was for "default"
-   * (the Hive JDBC driver's fallback) and the backend had no such database.
+   * The database the client requested via `USE_DATABASE` at session open. Used by
+   * `JdbcOperationManager` as the metadata schema filter. `None` when no `USE_DATABASE`
+   * was requested, or when the request was for "default" (the Hive JDBC driver's fallback)
+   * and the dialect's `setSchema` call failed.
    */
   private[jdbc] var effectiveDatabase: Option[String] = None
 
@@ -78,10 +79,8 @@ class JdbcSessionImpl(
       sessionConf.get(ENGINE_JDBC_SESSION_INITIALIZE_SQL))
     conf.get(USE_DATABASE).foreach { database =>
       try {
-        if (JdbcDialects.get(sessionConf).setCurrentDatabase(sessionConnection, database)) {
-          effectiveDatabase = Some(database)
-          info(s"Switched to database: $database")
-        }
+        JdbcDialects.get(sessionConf).setSchema(sessionConnection, database)
+        effectiveDatabase = Some(database)
       } catch {
         case NonFatal(e) if database == "default" =>
           // The Hive JDBC driver sends "default" when the user didn't specify a database