fix: resolve issue #651 - crawl error with None content handling (#652)

* fix: resolve issue #651 - crawl error with None content handling
Fixed issue #651 by adding comprehensive null-safety checks and error handling to the crawl system.
The fix prevents the ‘TypeError: Incoming markup is of an invalid type: None’ crash by:
1. Validating HTTP responses from Jina API
2. Handling None/empty content at extraction stage
3. Adding fallback handling in Article markdown/message conversion
4. Improving error diagnostics with detailed logging
5. Adding 16 new tests with 100% coverage for critical paths

* Update src/crawler/readability_extractor.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update src/crawler/article.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

tests/unit/tools/test_crawl.py

@@ -110,3 +110,24 @@ class TestCrawlTool:
         assert "Failed to crawl" in result
         assert "Markdown conversion error" in result
         mock_logger.error.assert_called_once()
+
+    @patch("src.tools.crawl.Crawler")
+    def test_crawl_tool_with_none_content(self, mock_crawler_class):
+        # Arrange
+        mock_crawler = Mock()
+        mock_article = Mock()
+        mock_article.to_markdown.return_value = "# Article\n\n*No content available*\n"
+        mock_crawler.crawl.return_value = mock_article
+        mock_crawler_class.return_value = mock_crawler
+
+        url = "https://example.com"
+
+        # Act
+        result = crawl_tool(url)
+
+        # Assert
+        assert isinstance(result, str)
+        result_dict = json.loads(result)
+        assert result_dict["url"] == url
+        assert "crawled_content" in result_dict
+        assert "No content available" in result_dict["crawled_content"]