fix(#141): use stdinValidateDocmap in Clean test — avoid real os.Stdin dependency

TestRunValidateDocmap_Clean was reading real os.Stdin (fragile in CI).
Switch to stdinValidateDocmap with a covered file and empty-stdin test
already covered by TestRunValidateDocmap_EmptyStdin.

cmd/review-bot/validatedocmap_test.go

@@ -60,19 +60,13 @@ mappings:
       - docs/foo.md
 `)
 
-	var code int
-	stdout, _ := captureOutput(func() {
-		code = runValidateDocmap([]string{
-			"--docmap", docmap,
-			"--repo-root", dir,
-		})
-	})
-	// Provide stdin via the global os.Stdin would require process tricks.
-	// The implementation reads os.Stdin directly; for tests we need a different approach.
-	// See TestRunValidateDocmap_WithStdin_* below which use a helper that replaces os.Stdin.
-	// This case: empty stdin (no changed files) → clean.
+	// A covered file with all docs existing → clean.
+	code, stdout, _ := stdinValidateDocmap(t,
+		"lib/foo/bar.ex\n",
+		[]string{"--docmap", docmap, "--repo-root", dir},
+	)
 	if code != 0 {
-		t.Errorf("expected exit 0 for clean (empty stdin), got %d", code)
+		t.Errorf("expected exit 0 for clean, got %d", code)
 	}
 	if !strings.Contains(stdout, "OK") {
 		t.Errorf("expected 'OK' in stdout, got %q", stdout)