Add structured validation failures

internal/validation/diagnostic_test.go

@@ -0,0 +1,49 @@
+package validation
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+)
+
+func TestRunPreservesCanceledGoSetup(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	err := Run(ctx, Request{RepositoryDir: t.TempDir()})
+	var failure *Failure
+	if !errors.As(err, &failure) {
+		t.Fatalf("Run() error = %v, want a validation failure", err)
+	}
+	if failure.Check != GoTest {
+		t.Fatalf("failure check = %q, want %q", failure.Check, GoTest)
+	}
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("Run() error = %v, want context cancellation preserved", err)
+	}
+}
+
+func TestFailureFormatsDiagnosticsAndUnwrapsCause(t *testing.T) {
+	wantCause := errors.New("underlying check error")
+	failure := &Failure{
+		Check: GoTest,
+		Findings: []Finding{{
+			Path:    "README.md",
+			Line:    4,
+			Column:  2,
+			Message: "broken destination",
+		}},
+		Measurement: &FileSizeMeasurement{Bytes: 12, LimitBytes: 10},
+		Cause:       wantCause,
+	}
+
+	if !errors.Is(failure, wantCause) {
+		t.Fatalf("Failure does not unwrap its cause")
+	}
+	for _, part := range []string{"go-test", "README.md:4:2", "broken destination", "12 bytes", "10 byte limit", wantCause.Error()} {
+		if !strings.Contains(failure.Error(), part) {
+			t.Errorf("Failure.Error() = %q; want it to include %q", failure.Error(), part)
+		}
+	}
+}