fix: revalidate when using CacheMode::NoCache

[beanow-at-crabnebula]

Fixes #173

Ensures we're using conditional_fetch as the documentation for NoCache suggests.
While keeping the forced Cache-Control: no-cache request header like the existing behavior.

Includes a regression test on http-cache-reqwest similar to surrounding tests there.

Note: not included in this PR is whether the NoCache comment should be updated to mention the no-cache header being set.

Additionally it doesn't mention that Cache-Control: no-cache is always set when using NoCache.
Which may be good to know when comparing with the Reload option that doesn't set it.

Assisted-by: claude-opus-5-5

[codecov]

Codecov Report

❌ Patch coverage is 97.46835% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.19%. Comparing base (b1c3fb4) to head (a0e6f69).
⚠️ Report is 52 commits behind head on main.

Files with missing lines	Patch %	Lines
http-cache/src/lib.rs	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #174      +/-   ##
==========================================
+ Coverage   85.87%   90.19%   +4.32%     
==========================================
  Files          19       19              
  Lines        7502    11470    +3968     
==========================================
+ Hits         6442    10345    +3903     
- Misses       1060     1125      +65     

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[beanow-at-crabnebula]

The failed CI check seems to be a test flake.

It's a first_duration < 50ms wall-clock budget for a test that involves disk I/O.
I couldn't locally reproduce that test to fail.