qapi: Assert built-in types exist

QAPISchema.lookup_type('FOO') returns a QAPISchemaType when type 'FOO'
exists, else None.  It won't return None for built-in types like
'int'.

Since mypy can't see that, it'll complain that we assign the
Optional[QAPISchemaType] returned by .lookup_type() to QAPISchemaType
variables.

Add assertions to help it over the hump.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240315152301.3621858-13-armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>

scripts/qapi/introspect.py

@@ -227,10 +227,14 @@ def _use_type(self, typ: QAPISchemaType) -> str:
 
         # Map the various integer types to plain int
         if typ.json_type() == 'int':
-            typ = self._schema.lookup_type('int')
+            type_int = self._schema.lookup_type('int')
+            assert type_int
+            typ = type_int
         elif (isinstance(typ, QAPISchemaArrayType) and
               typ.element_type.json_type() == 'int'):
-            typ = self._schema.lookup_type('intList')
+            type_intList = self._schema.lookup_type('intList')
+            assert type_intList
+            typ = type_intList
         # Add type to work queue if new
         if typ not in self._used_types:
             self._used_types.append(typ)