[hive] Fix malformed error message for unknown client pool cache key

[thswlsqls]

Purpose

fix #8638

• CachedClientPool.extractKey() threw new RuntimeException("Unknown key element %s" + trimmed); RuntimeException(String) does no %s substitution, so users saw a garbled message like Unknown key element %sconf.
• Reached on a normal misconfiguration: client-pool-cache.keys=conf (missing the conf: prefix) hits the switch default.
• Drop the literal %s and concatenate trimmed directly. Sibling checks (lines 162-165, 172-175) use Preconditions.checkArgument where %s is a real placeholder.

Tests

• Added TestCachedClientPool#testExtractKeyUnknownElementErrorMessage asserting the message is exactly Unknown key element conf (proves the literal %s is gone).
• mvn -pl paimon-hive/paimon-hive-catalog clean install (JDK 11) passed: checkstyle, spotless, rat, and TestCachedClientPool (14 tests) green.