Unify DOLT_DIFF CLI and SQL table function logic

[elianddb]

The DOLT_DIFF implementation currently duplicates core behavior between the CLI (dolt/go/cmd/dolt/commands/diff.go) and the SQL table function (dolt/go/libraries/doltcore/sqle/dtablefunctions/dolt_diff.go).

We should extract shared helpers that parse and normalize diff options. Consolidation will reduce maintenance overhead, and prevent subtle inconsistencies in flags and error handling.

For further context visit #9790.