[core] Reuse manifest merge results on commit retry

[leaves12138]

Summary

• Reuse the previous manifest merge result on commit retry when all previously merged input manifests still exist in the latest snapshot.
• Replace those input manifests in the latest manifest list with the merged output manifests, preserving concurrent manifests and falling back to a fresh merge if any input manifest disappeared.
• Always write a fresh base manifest list for the retry commit so no base manifest list file is reused across attempts.
• Add retry conflict tests for contiguous, non-contiguous, missing-before, multi-retry, empty-table, and delete-order cases.

Tests

• mvn -pl paimon-core -Pfast-build -Dtest=FileStoreCommitTest#testCommitRetryReusePreviousManifestMergeResultWhenBeforeStillExists+testCommitRetryReusePreviousManifestMergeResultWhenBeforeExistsNonContiguously+testCommitRetryDoesNotReusePreviousManifestMergeResultWhenBeforeWasMerged+testCommitRetryReusePreviousManifestMergeResultAcrossMultipleRetries+testCommitRetryFromEmptyTableWithConcurrentFirstSnapshot+testCommitRetryReusePreviousManifestMergeResultPreservesDeleteOrder test
• mvn -pl paimon-core -Pfast-build -Dtest=FileStoreCommitTest test
• mvn -pl paimon-core -DskipTests compile
• git diff --check