[KYUUBI #7305][FOLLOWUP] Skip Hive protocol "default" stub in session open

The Hive JDBC driver sends `USE_DATABASE = "default"` when the user did not
specify a database in the connection URL — a protocol-level stub
indistinguishable from a genuine request for a database literally named
"default". Forwarding that into `setSchema` plus recording it as
`effectiveDatabase` caused PG and ClickHouse metadata operations to scope to
a non-existent or unintended schema, breaking the existing
`get tables` suites in CI.

Filter `"default"` at the session entry point: skip both the dialect
`setSchema` call and the `effectiveDatabase` assignment for it. Drops the
prior try/catch that only existed to tolerate the failure path. Also rewords
the in-tree `MySQLDialect.setSchema` comment to describe the externally
observed behavior of the documented `databaseTerm` connection property,
without paraphrasing driver internals.

URL paths that name a real non-`default` database keep the new scoping
behavior; URL paths that explicitly name `default` continue to behave as
before this PR (no metadata scope), which is the same as URL paths with no
database — there is no observable regression.

Author: wangzhigang1999

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

@@ -151,11 +151,12 @@ class MySQLDialect extends JdbcDialect {
   }
 
   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.
+    // The MySQL Connector/J `databaseTerm` connection property selects which of
+    // setCatalog / setSchema actually switches the session database; the other
+    // call returns without effect for that mode (verified by black-box testing
+    // and consistent with the reference manual). Invoking both ensures the
+    // database is switched regardless of how the user configured the URL.
+    // StarRocksDialect / DorisDialect inherit this via MySQLDialect.
     conn.setCatalog(schema)
     conn.setSchema(schema)
   }

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

@@ -19,7 +19,6 @@ package org.apache.kyuubi.engine.jdbc.session
 import java.sql.{Connection, DatabaseMetaData}
 
 import scala.util.{Failure, Success, Try}
-import scala.util.control.NonFatal
 
 import org.apache.kyuubi.KyuubiSQLException
 import org.apache.kyuubi.config.KyuubiConf
@@ -50,8 +49,10 @@ class JdbcSessionImpl(
   /**
    * 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.
+   * was requested, or when the request was the literal `"default"`. The Hive JDBC
+   * driver sends `"default"` as a protocol stub when the user did not specify a
+   * database in the URL, so it cannot be distinguished from a genuine request and
+   * is treated as "no scope".
    */
   private[jdbc] var effectiveDatabase: Option[String] = None
 
@@ -77,17 +78,13 @@ class JdbcSessionImpl(
       sessionConf,
       sessionConnection,
       sessionConf.get(ENGINE_JDBC_SESSION_INITIALIZE_SQL))
-    conf.get(USE_DATABASE).foreach { database =>
-      try {
-        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
-          // in the connection URL. Tolerate USE failure in that case so the session can
-          // still open against backends that have no "default" database.
-          warn(s"Failed to switch to database 'default', ignored.", e)
-      }
+    // The Hive JDBC driver sends `"default"` as USE_DATABASE when the user did not
+    // specify a database in the URL: a protocol stub indistinguishable from a real
+    // request. Filter it here so we don't push a non-existent / unintended schema
+    // filter into metadata operations on backends without a `default` database.
+    conf.get(USE_DATABASE).filter(_ != "default").foreach { database =>
+      JdbcDialects.get(sessionConf).setSchema(sessionConnection, database)
+      effectiveDatabase = Some(database)
     }
     super.open()
     info(s"The jdbc session is started.")