[henriquejsza]

Purpose

Closes #10883.

All branches of a table share the same physical data, manifest, index and
statistics files; only snapshot, tag and schema metadata are branch scoped
(FileStorePathFactory roots data and manifest paths at the table root, while
SnapshotManager/TagManager prepend the branch path). Snapshot expiration on
a branch built its skipping set only from that branch's own snapshots and tags
(ExpireSnapshotsImpl#collectTaggedSnapshots and manifestSkippingSet), so
when a branch created from a tag drops its own copy of that tag and then
expires, it deletes files that the main branch still references. After that,
reading main fails with FileNotFoundException on a manifest list or data
file, exactly as reported in the issue.

This change makes expiration aware of the other branches. Before deleting
files, AbstractFileStoreTable#newExpireSnapshots collects the data files and
the manifest/manifest-list/index/statistics files referenced by every other
branch's live snapshots and tags, and ExpireSnapshotsImpl skips any file that
is still referenced. The data-file lookup is restricted to the current
expiration batch so memory stays proportional to the batch, mirroring the
existing per-tag skipper. Reading another branch is best effort: if it fails,
the batch is kept rather than risking deletion of in-use files. When the table
has no other branch, the collector is null and the common path is unchanged.

Tests

• Added AppendOnlySimpleTableTest#testExpireOnBranchKeepsFilesReferencedByMain,
  which reproduces the issue: tag main's snapshot, create a branch from the tag,
  write on the branch, delete the branch's copy of the tag, expire the branch to
  one snapshot, then assert main is still fully readable. It fails on master
  (FileNotFoundException on main's manifest list) and passes with this change.
• mvn -pl paimon-core -Dtest='AppendOnlySimpleTableTest,ExpireSnapshotsTest,FileDeletionTest' test
  passes.
• mvn -pl paimon-core spotless:check checkstyle:check passes.