chore: resolve SA5011 staticcheck warnings in test files (#364)

Replace `if x == nil { t.Fatal(...) }` guard pattern with
`require.NotNil(t, x, ...)`. The old pattern confuses newer staticcheck
(Go 1.26+) which doesn't track `testing.T.Fatal` as noreturn in all
cases, flagging subsequent dereferences as possible nil dereferences.

Files already import testify/require so this is a minimal mechanical
change with no behavior difference — `require.NotNil` calls
`t.FailNow()` under the hood.

Affected:
- differ/differ_test.go (1 site)
- validator/refs_test.go (1 site)
- validator/validator_test.go (3 sites)

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: erraggy

differ/differ_test.go

@@ -10,9 +10,7 @@ import (
 
 func TestDifferNew(t *testing.T) {
 	d := New()
-	if d == nil {
-		t.Fatal("Expected non-nil Differ")
-	}
+	require.NotNil(t, d, "Expected non-nil Differ")
 
 	if d.Mode != ModeSimple {
 		t.Errorf("Expected default mode to be ModeSimple, got %d", d.Mode)

validator/refs_test.go

@@ -281,9 +281,7 @@ components:
 				t.Fatalf("Validate failed: %v", err)
 			}
 
-			if result == nil {
-				t.Fatal("Expected result, got nil")
-			}
+			require.NotNil(t, result, "Expected result, got nil")
 
 			hasRefError := false
 			for _, validationErr := range result.Errors {

validator/validator_test.go

@@ -15,9 +15,7 @@ import (
 // TestValidatorNew tests the New constructor
 func TestValidatorNew(t *testing.T) {
 	v := New()
-	if v == nil {
-		t.Fatal("New() returned nil")
-	}
+	require.NotNil(t, v, "New() returned nil")
 	if !v.IncludeWarnings {
 		t.Error("Expected IncludeWarnings to be true by default")
 	}
@@ -374,9 +372,7 @@ func TestCircularSchemaReferences(t *testing.T) {
 	}
 
 	// Validation should not crash on circular schemas
-	if result == nil {
-		t.Fatal("Expected result, got nil")
-	}
+	require.NotNil(t, result, "Expected result, got nil")
 
 	t.Logf("Circular schema validation completed with %d errors, %d warnings", result.ErrorCount, result.WarningCount)
 }
@@ -394,9 +390,7 @@ func TestDeeplyNestedSchemas(t *testing.T) {
 	}
 
 	// Validation should complete without stack overflow
-	if result == nil {
-		t.Fatal("Expected result, got nil")
-	}
+	require.NotNil(t, result, "Expected result, got nil")
 
 	t.Logf("Deeply nested schema validation completed with %d errors, %d warnings", result.ErrorCount, result.WarningCount)
 }