[core] Ensuring JDBC connection is not stale before using it

[junmuz]

Purpose

This change ensure connection is valid before it is used for locking or cataloging purposes. Changes made include

1. paimon-common/.../client/ClientPool.java

• Added a reconnect(C client) hook in ClientPoolImpl that subclasses can override
• Called it in run() before executing the action — the reassignment to client ensures the valid connection is what goes back into the deque via addFirst

2. paimon-core/.../jdbc/JdbcClientPool.java

• Stored connectionSupplier as a field for creating fresh connections
• Overrode reconnect() to validate via isClosed() and isValid(5), replacing stale connections
• Added closeQuietly() helper for safe cleanup of dead connections

3. paimon-core/.../jdbc/JdbcClientPoolTest.java (new)

• testValidConnectionIsReused — a healthy connection is returned as-is from the pool (same object identity)
• testClosedConnectionIsReplaced — a closed connection triggers creation of a fresh one
• testReplacedConnectionIsReturnedToPool — the fresh replacement goes back into the pool and is reused on subsequent calls
• testActionIsExecutedOnValidConnection — after replacing a stale connection, the action can execute real SQL successfully

Issue: 7474

Tests

There are new tests added that will ensure the change works as expected

API and Format

N/A

Documentation

No

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[junmuz]

@JingsongLi can you review the PR? The details are mentioned in the issue and there are test cases for covering the scenario.