fix(db2): explicitly register DB2Interval in TRANSFORMS for sqlglot 30

sqlglot 30 changed how the generator dispatch is built. In 28 the
auto-discovery walked all expression subclasses; in 30 it only
considers classes registered in sqlglot.expressions.EXPR_CLASSES,
which is populated once at module load via subclasses(__name__, Expr)
— i.e. ONLY built-in expression types. Custom user-defined
Expression subclasses defined outside that module — like our
DB2Interval — no longer get auto-wired to their <name>_sql handler.

Adding DB2Interval to TRANSFORMS as a lambda → db2interval_sql
restores the dispatch. All 24 dialects/db2_tests.py tests pass.

superset/sql/dialects/db2.py

@@ -136,6 +136,12 @@ class DB2(Postgres):
 
         TRANSFORMS = {
             **Postgres.Generator.TRANSFORMS,
+            # sqlglot 30 only auto-discovers <Name>_sql handlers for expression
+            # classes that live in sqlglot.expressions.EXPR_CLASSES (the
+            # registry is built once at module load via ``subclasses(__name__, Expr)``).
+            # Custom Expression subclasses defined outside that module — like
+            # DB2Interval below — must be wired up explicitly in TRANSFORMS.
+            DB2Interval: lambda self, e: self.db2interval_sql(e),
         }
 
         def db2interval_sql(self, expression: DB2Interval) -> str: