test: 补充缺失的核心模块测试，统一CLI测试行为

新增测试文件：
- tests/test_core/test_parser.py - 测试 parse_input/process_content/output_result
- tests/test_core/test_markdown_extra.py - 测试 extract_title_content/search_markdown
- tests/test_utils/test_encoding_detection.py - 测试编码检测模块
- tests/test_readers/test_html_downloader.py - 测试HTML下载器

修改：
- tests/conftest.py - 移除pytest.skip()，所有CLI测试在缺少依赖时直接失败（与HTML测试行为一致）

tests/test_readers/test_html_downloader.py

@@ -0,0 +1,43 @@
+"""测试 HTML 下载器模块。"""
+
+import pytest
+from unittest.mock import patch, MagicMock
+
+from readers.html.downloader import download_html
+from readers.html.downloader import pyppeteer, selenium, httpx, urllib
+
+
+class TestDownloadHtml:
+    """测试 download_html 统一入口函数。"""
+
+    def test_download_html_module_importable(self):
+        """测试 download_html 函数可以正常导入和调用。"""
+        # 只要不抛异常就可以
+        assert callable(download_html)
+
+    def test_downloaders_available(self):
+        """测试各下载器模块可用。"""
+        assert callable(pyppeteer.download)
+        assert callable(selenium.download)
+        assert callable(httpx.download)
+        assert callable(urllib.download)
+
+
+class TestIndividualDownloaders:
+    """测试单个下载器模块。"""
+
+    def test_pyppeteer_download_callable(self):
+        """测试 pyppeteer.download 可以调用。"""
+        assert callable(pyppeteer.download)
+
+    def test_selenium_download_callable(self):
+        """测试 selenium.download 可以调用。"""
+        assert callable(selenium.download)
+
+    def test_httpx_download_callable(self):
+        """测试 httpx.download 可以调用。"""
+        assert callable(httpx.download)
+
+    def test_urllib_download_callable(self):
+        """测试 urllib.download 可以调用（标准库）。"""
+        assert callable(urllib.download)