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>

src/crawler/jina_client.py

@@ -23,4 +23,11 @@ class JinaClient:
             )
         data = {"url": url}
         response = requests.post("https://r.jina.ai/", headers=headers, json=data)
+        
+        if response.status_code != 200:
+            raise ValueError(f"Jina API returned status {response.status_code}: {response.text}")
+        
+        if not response.text or not response.text.strip():
+            raise ValueError("Jina API returned empty response")
+        
         return response.text