test(server): harden raw trailer read on Windows

[quinnj]

Summary

• add an opt-in first-byte wait for raw HTTP test probes
• use it for the chunked trailers stream test that previously observed an empty raw response on Windows

Notes

The #1261 Windows failure saw raw == "" in the trailers assertion path. This keeps normal raw probes unchanged, but lets response-sensitive probes avoid treating a pre-response empty read as completion.

Validation

• reran feat(websockets): add isready(ws) to poll for buffered messages #1261 CI attempt 2: Windows Julia 1 passed, full GitHub Actions run passed
• PR test(server): harden raw trailer read on Windows #1270 GitHub Actions run: all workflow jobs passed, including both Windows jobs; codecov/patch passed, codecov/project remains red
• julia --project=. --startup-file=no --history-file=no test/http_server_http1_tests.jl
• git diff --check

Refs #1261

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.37%. Comparing base (83f4873) to head (06db385).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1270      +/-   ##
==========================================
- Coverage   84.51%   84.37%   -0.15%     
==========================================
  Files          28       28              
  Lines       10690    10690              
==========================================
- Hits         9035     9020      -15     
- Misses       1655     1670      +15     

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