Read complete HTTP headers in browser 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 browser test servers could reply after reading only the first TCP packet. A scoped header split across packets was then recorded as absent, weakening the header-isolation assertions.

Read complete headers in the existing TCP fixtures, share the bounded reader with the CORS fixture, and close pending reads when a fixture is disposed. Two regressions send headers in separate writes and verify that the server waits for the complete header. The focused checks live beside the shared fixture helper.

The final Linux PDF/browser class passes all 201 tests. The two fragmented-header regressions, redirect header-isolation checks and source-structure gate pass on each Windows .NET target. The browser security assertions remain unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.12%. Comparing base (7aa340e) to head (2810fbd).

Additional details and impacted files

@@                  Coverage Diff                  @@
##           v2-speedygonzales     #494      +/-   ##
=====================================================
- Coverage              58.13%   58.12%   -0.01%     
=====================================================
  Files                    484      484              
  Lines                  36063    36063              
  Branches                7256     7256              
=====================================================
- Hits                   20964    20963       -1     
  Misses                 12726    12726              
- Partials                2373     2374       +1     

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