[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.
• If a retry cannot reuse that merge result, skip ManifestFileMerger.merge and write the latest snapshot data manifests directly as the new base manifest list.
• Avoid caching the skipped no-op path as a reusable merge result for later retries.
• Always write a fresh base manifest list for the retry commit, and 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+testCommitRetrySkipsManifestMergeWhenPreviousMergeCannotBeReused+testCommitRetryReusePreviousManifestMergeResultAcrossMultipleRetries+testCommitRetryFromEmptyTableWithConcurrentFirstSnapshot+testCommitRetryReusePreviousManifestMergeResultPreservesDeleteOrder test
• mvn -pl paimon-core -Pfast-build -Dtest=FileStoreCommitTest test
• mvn -pl paimon-core -DskipTests compile
• git diff --check