[core] Make LocalKvDb flush failure-safe

[JingsongLi]

Purpose

Make LocalKvDb flush failure-safe.

Changes

• Keep the memtable intact until the SST has been written and published to level 0.
• Remove an incomplete SST when writing or closing the writer fails.
• Add a regression test that injects a flush failure and verifies that reads and a later retry still succeed.

Verification

• mvn -q -pl paimon-common -Pfast-build -DwildcardSuites=none '-Dtest=LocalKvDbTest#testFlushFailureKeepsMemTableForRetry+testFlushToSst+testCloseFlushesMemTable' test
• mvn -q -pl paimon-common spotless:check -DskipTests
• mvn -q -pl paimon-common -DskipTests compile

[leaves12138]

Looks good. The MemTable is retained until the SST is fully written and published, and failed writers/files are cleaned up without losing retryability. LocalKvDbTest passes locally.