[core] Prevent tag deletion when referenced by branches

[ArnavBalyan]

Purpose

• A branch created from a tag shares the tag snapshot's data and manifest files. Deleting the tag makes the branch unqueryable.
• Block deleteTag with an error with referencing branches, so a tag cannot be removed while a branch depends on it.
• Make automatic tag expiration skip such tags with a warning instead, since it runs during commit and must not fail the commit.
• Supported for FileSystemBranchManager and CatalogBranchManager.

Tests

• UT