refactor: delete the unreachable None branch in retry-after parsing - #141

What

Delete the if parsed is None: return None branch (and its # pragma: no cover) in _parse_retry_after, src/httpware/middleware/resilience/retry.py.

Why

email.utils.parsedate_to_datetime cannot return None on any interpreter httpware supports (requires-python >=3.11), so the branch was dead code that coverage had to be told to ignore.

Evidence

• CPython source (inspect.getsource on 3.11.16, 3.12.13, 3.13.15, 3.14.7): the function has exactly two return statements, and both return a datetime.datetime(...). When _parsedate_tz yields None it does raise ValueError('Invalid date value or format ...'). 3.13+ also turns OverflowError from the datetime constructor into ValueError.
• CPython docs (email.utils): the function "on success returns a datetime; otherwise ValueError is raised".
• Runtime probe on 3.11, 3.12, 3.13, 3.14 and 3.14t with 13 malformed or edge-case inputs ("", "garbage", day 32, a bad month, hour 25, NUL, a huge year, a missing or unknown zone, -0000). Every call either raised or returned a datetime, and none returned None.

Checks (local)

• just install: no lockfile change
• just lint-ci: pass
• just test-ci: 819 passed, coverage 100.00%