fix(api): use raw token for Vikunja Authorization header

Vikunja's API expects the raw API token in the Authorization header
without the "Bearer" prefix. Using "Bearer <token>" causes HTTP 401
errors, which broke the auto-merge preconditions check in CI.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

tests/unit/test_api_clients.py

@@ -606,7 +606,7 @@ class TestVikunjaClient:
     def test_init_sets_headers(self) -> None:
         client = VikunjaClient("https://work.example.com", "tok")
         assert client._base_url == "https://work.example.com"
-        assert client._session.headers["Authorization"] == "Bearer tok"
+        assert client._session.headers["Authorization"] == "tok"
 
     def test_list_tasks(self) -> None:
         client = VikunjaClient("https://work.example.com", "tok")