🚀 CRITICAL FIX: Resolve GitHub diff API header issue

🐛 ROOT CAUSE IDENTIFIED:
- GitHub API was returning JSON metadata instead of diff content
- Session headers were set to 'application/vnd.github.v3+json'
- This caused 16,951 characters of PR JSON data instead of actual diff

🔧 SOLUTION:
- Override Accept header in get_pr_diff() method specifically
- Use 'application/vnd.github.v3.diff' for diff requests
- Maintain JSON format for other API calls

✅ EXPECTED RESULT:
- Should now receive actual Git diff content
- Unidiff parser will find files to process
- Code review will work as intended

This resolves the core issue where PatchSet was empty despite having
'content' - we were parsing JSON instead of diff format.

Author: truongnh1992

gemini_reviewer/github_client.py

@@ -175,8 +175,13 @@ def get_pr_diff(self, owner: str, repo: str, pull_number: int) -> str:
             # Use direct API call for diff
             api_url = f"{self.config.api_base_url}/repos/{repo_name}/pulls/{pull_number}.diff"
 
+            # Override Accept header to specifically request diff format
+            diff_headers = {
+                'Accept': 'application/vnd.github.v3.diff'
+            }
+            
             logger.debug(f"Making diff API request to: {api_url}")
-            response = self._session.get(api_url, timeout=self.config.timeout)
+            response = self._session.get(api_url, headers=diff_headers, timeout=self.config.timeout)
 
             if response.status_code == 200:
                 diff = response.text