[Fusion] Fixed satisfiability issue with fields on the Query type

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Satisfiability/RequirementsValidator.cs

@@ -16,7 +16,7 @@ internal sealed class RequirementsValidator(
     public ImmutableArray<SatisfiabilityError> Validate(
         SelectionSetNode requirements,
         MutableObjectTypeDefinition contextType,
-        SatisfiabilityPathItem parentPathItem,
+        SatisfiabilityPathItem? parentPathItem,
         string excludeSchemaName,
         SatisfiabilityPath? cycleDetectionPath = null)
     {
@@ -384,12 +384,17 @@ internal sealed class RequirementsValidatorContext
 {
     public RequirementsValidatorContext(
         MutableObjectTypeDefinition contextType,
-        SatisfiabilityPathItem parentPathItem,
+        SatisfiabilityPathItem? parentPathItem,
         string excludeSchemaName,
         SatisfiabilityPath? cycleDetectionPath = null)
     {
         TypeContext.Push(contextType);
-        Path.Push(parentPathItem);
+
+        if (parentPathItem is not null)
+        {
+            Path.Push(parentPathItem);
+        }
+
         ExcludeSchemaName = excludeSchemaName;
         CycleDetectionPath = cycleDetectionPath ?? [];
     }

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/SatisfiabilityValidator.cs

@@ -162,7 +162,7 @@ private void VisitOutputField(
                     _requirementsValidator.Validate(
                         requirements,
                         type,
-                        context.Path.Peek(),
+                        previousPathItem,
                         excludeSchemaName: schemaName);
 
                 if (requirementErrors.Length != 0)

src/HotChocolate/Fusion-vnext/test/Fusion.Composition.Tests/SatisfiabilityValidatorTests.cs

@@ -2536,6 +2536,39 @@ type Section {
             """);
     }
 
+    [Fact]
+    public void RootRequirement()
+    {
+        // arrange
+        var merger = new SourceSchemaMerger(
+            CreateSchemaDefinitions(
+            [
+                """
+                # Schema A
+                type Query {
+                    optionalField: String
+                }
+                """,
+                """
+                # Schema B
+                type Query {
+                    requiredField(value: String @require(field: "optionalField")): String!
+                }
+                """
+            ]),
+            new SourceSchemaMergerOptions { AddFusionDefinitions = false });
+
+        var schema = merger.Merge().Value;
+        var log = new CompositionLog();
+        var satisfiabilityValidator = new SatisfiabilityValidator(schema, log);
+
+        // act
+        var result = satisfiabilityValidator.Validate();
+
+        // assert
+        Assert.True(result.IsSuccess);
+    }
+
     [Theory]
     [MemberData(nameof(GlobalObjectIdentificationExamplesData))]
     public void GlobalObjectIdentification_Examples(string[] sdl, bool success, string? logs = null)