Correct HTTP response-limit and cancellation test fixtures

[PrzemyslawKlys]

These fixture changes are included in #487. Closing this duplicate so the browser correction and its test fixtures can be reviewed together.

The stalled-response checks finish without waiting for the test runner's synchronization context. Timeout and caller cancellation remain separate cases for each HTTP consumer, with the existing deadlines, watchdogs, exception assertions and response-disposal checks.

The response-limit fixtures distinguish declared and unknown-length bodies. The latter use a non-seekable stream and verify that Content-Length is absent. The declared-size cases exercise the default limit without allocating four unused 16 MiB bodies.

The public crawl fixture also runs in Windows PowerShell 5.1: its embedded server uses supported C# syntax, and relative-link expectations use URI APIs available in both runtimes.

Validation: 39 focused checks passed on .NET 10 and .NET 8, and 35 on .NET Framework 4.7.2. All 25 public crawl Pester cases passed in PowerShell 5.1 and 7. The last full local .NET 10 run passed 1,545 cases and failed one Firefox navigation case; browser startup and document capture are addressed separately in #487. Current PR CI remains a merge gate.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.12%. Comparing base (7aa340e) to head (7784ca8).
⚠️ Report is 1 commits behind head on v2-speedygonzales.

Additional details and impacted files

@@                  Coverage Diff                  @@
##           v2-speedygonzales     #496      +/-   ##
=====================================================
- Coverage              58.13%   58.12%   -0.02%     
=====================================================
  Files                    484      486       +2     
  Lines                  36063    36054       -9     
  Branches                7256     7249       -7     
=====================================================
- Hits                   20964    20955       -9     
  Misses                 12726    12726              
  Partials                2373     2373              

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