Restore full branch coverage for DeferStreamDirectiveOnRootField rule

Replace the unreachable final isinstance branch with an else clause and
add tests for the previously uncovered branches (unrelated directives on
a root field, undefined fragment spreads, inline fragments without defer,
and operations whose schema lacks a matching root type).

Author: Cito

src/graphql/validation/rules/defer_stream_directive_on_root_field.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, cast
 
 from ...error import GraphQLError
 from ...language import (
@@ -103,8 +103,9 @@ def forbid_defer_stream(
                         visited_fragments,
                     )
                 visited_fragments.add(fragment_name)
-            elif isinstance(selection, InlineFragmentNode):
-                defer = get_directive(selection, GraphQLDeferDirective.name)
+            else:  # the only remaining selection kind is an inline fragment
+                inline_fragment = cast("InlineFragmentNode", selection)
+                defer = get_directive(inline_fragment, GraphQLDeferDirective.name)
                 if defer:
                     self.report_error(
                         GraphQLError(
@@ -117,6 +118,6 @@ def forbid_defer_stream(
                     operation_type,
                     root_type,
                     fragments,
-                    selection.selection_set,
+                    inline_fragment.selection_set,
                     visited_fragments,
                 )

tests/validation/test_defer_stream_directive_on_root_field.py

@@ -341,3 +341,57 @@ def stream_field_on_fragment_on_root_subscription_field():
                 },
             ],
         )
+
+    def other_directive_on_root_mutation_field():
+        assert_valid(
+            """
+            mutation {
+              mutationField @foo {
+                body
+              }
+            }
+            """
+        )
+
+    def defer_fragment_spread_on_undefined_fragment():
+        assert_valid(
+            """
+            mutation {
+              ...undefinedFragment @defer
+            }
+            """
+        )
+
+    def inline_fragment_without_defer_on_root_mutation_field():
+        assert_valid(
+            """
+            mutation {
+              ... {
+                mutationField {
+                  body
+                }
+              }
+            }
+            """
+        )
+
+    def defer_on_mutation_without_mutation_root_type():
+        query_only_schema = build_schema(
+            """
+            type Query {
+              message: String
+            }
+            """
+        )
+        assert_validation_errors(
+            DeferStreamDirectiveOnRootField,
+            """
+            mutation {
+              ... @defer {
+                message
+              }
+            }
+            """,
+            [],
+            schema=query_only_schema,
+        )