Assign datetime.replace() return value in validate_file

[bysiber]

validate_file() calls datetime.replace(tzinfo=timezone.utc) to normalize tz-naive datetimes before comparing last_modified and if_modified_since timestamps. However, datetime.replace() returns a new datetime — it doesn't mutate in place. Since the return values aren't assigned back, both variables remain tz-naive after the "fix."

The practical effect: on any server not running in UTC, last_modified.timestamp() interprets the tz-naive datetime as local time while the tz-aware one is interpreted as UTC. This produces incorrect comparisons and breaks HTTP 304 Not Modified responses — either serving stale content or forcing unnecessary re-downloads depending on the timezone offset direction.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.817%. Comparing base (785d77f) to head (371533d).

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #3141       +/-   ##
=============================================
+ Coverage   87.793%   87.817%   +0.024%     
=============================================
  Files          105       105               
  Lines         8143      8143               
  Branches      1290      1290               
=============================================
+ Hits          7149      7151        +2     
+ Misses         687       686        -1     
+ Partials       307       306        -1     

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