Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@vera_bench/prompts.py`:
- Around line 24-28: Add a regression test that verifies _fetch_url() sends the
custom User-Agent header: write a unit test that patches or mocks
urllib.request.urlopen (or urllib.request.Request) and captures the Request
passed into urlopen when calling _fetch_url(url). Assert that the captured
Request.headers (or Request.get_header("User-Agent")) equals the module constant
_USER_AGENT; reference the _fetch_url function and the _USER_AGENT symbol to
locate where to call and what to assert, and ensure the mock returns an object
with a read().decode("utf-8") result to simulate a normal response.
- Line 21: The constant _USER_AGENT lacks an explicit type hint; update its
declaration (the _USER_AGENT symbol) to include a Python type annotation (str)
per repository typing rules (Python 3.11+), e.g., annotate _USER_AGENT: str so
the constant is explicitly typed as a string.