[Fusion] Fix composition crash on unimplemented merged interface field

src/HotChocolate/Fusion/src/Fusion.Composition/PostMergeValidationRules/ImplementedByInaccessibleRule.cs

@@ -52,7 +52,14 @@ private static void Handle(
 
             foreach (var interfaceField in accessibleInterfaceFields)
             {
-                var field = type.Fields[interfaceField.Name];
+                // An implementing type can be missing fields that were contributed to a merged
+                // interface by another source schema. That missing-field case is validated
+                // separately (e.g. by InterfaceFieldNoImplementationRule for object types), so we
+                // skip it here to avoid throwing when looking up the field on the implementing type.
+                if (!type.Fields.TryGetField(interfaceField.Name, out var field))
+                {
+                    continue;
+                }
 
                 if (field.HasFusionInaccessibleDirective()
                     || type.HasFusionInaccessibleDirective())

src/HotChocolate/Fusion/test/Fusion.Composition.Tests/PostMergeValidationRules/ImplementedByInaccessibleRuleTests.cs

@@ -150,4 +150,34 @@ interface User implements Node {
                 """
             ]);
     }
+
+    // When two source schemas declare the same-named interface with different fields, the merged
+    // interface holds the union of those fields, so an implementing type does not necessarily
+    // provide every field of the interface. This rule must not assume the field exists on the
+    // implementing type; the unimplemented field is reported by InterfaceFieldNoImplementationRule.
+    [Fact]
+    public void Validate_MergedInterfaceFieldNotOnAllImplementers_Succeeds()
+    {
+        AssertValid(
+        [
+            """
+            interface Error {
+                code: String!
+            }
+
+            type ErrorWithCode implements Error {
+                code: String!
+            }
+            """,
+            """
+            interface Error {
+                message: String!
+            }
+
+            type ErrorWithMessage implements Error {
+                message: String!
+            }
+            """
+        ]);
+    }
 }