reject path traversal in files.read and files.ls (T-78)

Both handlers concatenated the request path onto the workspace root
without validating containment, letting `path: "../../../etc/passwd"`
escape the workspace. resolveUnderRoot normalizes the path and
checks containment under root.absolute.path before any filesystem
access.

Co-Authored-By: Claude <noreply@anthropic.com>

test/files/path_safety_test.dart

@@ -0,0 +1,66 @@
+import 'dart:io';
+
+import 'package:clide/src/files/path_safety.dart';
+import 'package:test/test.dart';
+
+void main() {
+  late Directory root;
+
+  setUp(() {
+    root = Directory.systemTemp.createTempSync('clide_path_safety_');
+  });
+
+  tearDown(() {
+    if (root.existsSync()) root.deleteSync(recursive: true);
+  });
+
+  group('resolveUnderRoot', () {
+    test('plain relative path resolves under root', () {
+      final out = resolveUnderRoot(root, 'file.txt');
+      expect(out, '${root.absolute.path}/file.txt');
+    });
+
+    test('nested relative path resolves under root', () {
+      final out = resolveUnderRoot(root, 'src/main.dart');
+      expect(out, '${root.absolute.path}/src/main.dart');
+    });
+
+    test('empty relative path resolves to root itself', () {
+      final out = resolveUnderRoot(root, '');
+      expect(out, root.absolute.path);
+    });
+
+    test('rejects ../etc/passwd traversal', () {
+      expect(() => resolveUnderRoot(root, '../../../etc/passwd'),
+          throwsA(isA<PathOutsideRoot>()));
+    });
+
+    test('rejects traversal that lands at filesystem root', () {
+      expect(() => resolveUnderRoot(root, '../'),
+          throwsA(isA<PathOutsideRoot>()));
+    });
+
+    test('rejects sibling-directory traversal', () {
+      expect(() => resolveUnderRoot(root, '../sibling/file'),
+          throwsA(isA<PathOutsideRoot>()));
+    });
+
+    test('allows internal `..` that stays under root', () {
+      final out = resolveUnderRoot(root, 'a/b/../c');
+      expect(out, '${root.absolute.path}/a/c');
+    });
+
+    test('rejects path that prefix-matches root but is outside', () {
+      // Sibling dir whose name starts with the root's last segment.
+      // resolveUnderRoot must not be fooled by string-prefix matching.
+      final twin = Directory('${root.parent.path}/${root.uri.pathSegments.where((s) => s.isNotEmpty).last}_twin');
+      try {
+        twin.createSync();
+        expect(() => resolveUnderRoot(root, '../${twin.uri.pathSegments.where((s) => s.isNotEmpty).last}/file'),
+            throwsA(isA<PathOutsideRoot>()));
+      } finally {
+        if (twin.existsSync()) twin.deleteSync(recursive: true);
+      }
+    });
+  });
+}