fix(http): evaluate cache validators for HEAD requests

[dennisimoo]

Fixes #7327.

Move the existing HEAD response below cache-validator evaluation, while keeping it before Range handling and file opening. Conditional HEAD requests now return 304 like GET, as required by RFC 9110 §13.2.2.

Tests cover matching ETags, *, modified-since dates, ETag precedence, ignored Range headers and serveDir(). Both regressions fail before the fix.

Tests: deno test -A --doc http/ — 479 passed, 987 steps, 2 ignored. Lint and formatting passed. Full-suite verification is memory-limited: the sibling branch's deno task ok was killed in an unrelated CBOR test allocating 4–8 GiB; this branch's full run was stopped before repeating that allocation.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.04%. Comparing base (01fe755) to head (db8bbe0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7329   +/-   ##
=======================================
  Coverage   95.04%   95.04%           
=======================================
  Files         618      618           
  Lines       51746    51746           
  Branches     9402     9402           
=======================================
  Hits        49181    49181           
  Misses       2022     2022           
  Partials      543      543           

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