fix: add missing scope validation in Session.wait_for_schema_agreement

The docstring and test promised ValueError for invalid scope values
(e.g. 'planet'), but the validation was never implemented in the method
body.  Add an explicit check against the three SchemaAgreementScope
members.

Introduced in commit 0d215f4 (cluster: add Session.wait_for_schema_agreement).

Author: mykaul

cassandra/cluster.py

@@ -3481,6 +3481,11 @@ def wait_for_schema_agreement(self, wait_time: Optional[float] = None,
         if wait_time is not None and wait_time <= 0:
             raise ValueError("wait_time must be greater than 0")
 
+        if scope not in (SchemaAgreementScope.RACK, SchemaAgreementScope.DC, SchemaAgreementScope.CLUSTER):
+            raise ValueError(
+                "scope must be SchemaAgreementScope.RACK, .DC, or .CLUSTER"
+            )
+
         total_timeout = wait_time if wait_time is not None else self.cluster.max_schema_agreement_wait
         if total_timeout <= 0:
             raise ValueError("total_timeout must be greater than 0")