Run shared transport behavior tests on every platform #1917

[Filed by Copilot on behalf of @bghgary]

Several existing transport tests skip entire platforms rather than only assertions tied to platform-specific diagnostics.

In UrlRequestErrorReporting.cpp, SKIP_WITHOUT_TRANSPORT_ERROR_DETAIL skips six tests entirely on Windows. This suppresses shared status, reuse, and cancellation checks along with diagnostic assertions. For example, AbortInterruptsInFlightRequest never checks whether cancellation completes within its timeout on Windows. MissingAppResourceReportsError is separately Apple-only.

Please separate shared behavior checks from backend-specific diagnostic expectations:

• Run failure-status, response-state, request-reuse, and cancellation checks on every supported backend.
• Keep platform-specific error-symbol and error-code assertions separate, so unsupported diagnostics do not suppress shared checks.
• Audit the remaining platform guards for the same pattern.

These exclusions predate the response-body fixes in PR #38; this issue tracks the broader existing coverage gap.