fix(test): subprocess 显式指定 encoding=utf-8，修复 Windows GBK 解码崩溃

_run_cli 辅助函数的 subprocess.run 用 text=True 但未指定 encoding，Windows 中文系统默认用 GBK 解码子进程输出，--help 的非 ASCII 字符（box-drawing/em-dash）触发 UnicodeDecodeError。显式指定 encoding='utf-8' 与脚本的 force_utf8_stdout() 对齐。

测试: 362/362 全部通过

tests/test_cli_e2e.py

@@ -242,12 +242,18 @@ def _run_cli(*args, env=None):
     Uses ``sys.executable`` so the same interpreter that runs pytest runs
     the CLI. cwd is pinned to PROJECT_ROOT so the script's config-file
     auto-discovery (./searxng.toml) behaves deterministically.
+
+    ``encoding="utf-8"`` is explicit because Windows Chinese systems default
+    to GBK for subprocess text decoding, which crashes on non-ASCII chars
+    in --help output (e.g. box-drawing chars, em-dashes). The script itself
+    already forces UTF-8 stdout via ``force_utf8_stdout()``, so this just
+    matches the decoding side.
     """
     full_env = {**os.environ, **(env or {})}
     cmd = [sys.executable, str(PROJECT_ROOT / "scripts" / "search.py")] + list(args)
     return subprocess.run(
         cmd, cwd=str(PROJECT_ROOT),
-        capture_output=True, text=True, timeout=30,
+        capture_output=True, text=True, encoding="utf-8", timeout=30,
         env=full_env,
     )