[spark] Fix required configuration check for Paimon SparkSession extensions

[lsm1]

Purpose

Fix false failures in Paimon required Spark configuration checks by making the check rely on the current SparkSession configuration (instead of a temporary/active SQLConf), so Spark SQL operations (including creating/querying views across session restarts) don’t incorrectly report missing spark.sql.extensions.

Tests

• Spark SQL manual verification:

     create table paimon.default.p1 (
         k int,
         v string
     ) USING paimon
     tblproperties (
         'primary-key' = 'k'
     );
     insert into table paimon.default.p1 select 1,'2';
     create view v2 as  select * from paimon.default.p1;
     --- restart spark-sql session
     select * from v2;

• UT: paimon-spark-ut added/updated cases for required conf checks with
  temporary SQLConf.

API and Format

Documentation

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