[Regstar2]

Problem

A real Windows PowerShell 5.1 E2E build on 2026-08-23 reached the new fresh UUP conversion-package download, then failed before the HTTP request was sent:

Не удается найти перегрузку для "FormUrlEncodedContent" и количества аргументов: "5".

The Direct network path calls Invoke-WibHttpRequestCore, which built a List<KeyValuePair<string,string>> and passed it through positional New-Object Net.Http.FormUrlEncodedContent($pairs). Windows PowerShell 5.1 enumerates that list and treats the five form entries as five constructor arguments instead of one IEnumerable<KeyValuePair<string,string>> argument.

Fix

• add a PS5.1 compatibility helper that wraps the list as a single -ArgumentList element with the unary comma;
• override only Invoke-WibHttpRequestCore form-content construction, preserving the existing Network Policy, proxy behavior, HTTP status handling, downloads and error taxonomy;
• load the compatibility layer immediately after the base Network.ps1 implementation;
• add functional Pester coverage that constructs the exact five-field UUP dump POST body and reads the generated form-encoded content.

Regression coverage

tests/NetworkPowerShell51Compat.Tests.ps1 verifies:

• the compatibility override loads after Network.ps1;
• a five-field form body creates one System.Net.Http.FormUrlEncodedContent object instead of a five-argument constructor call;
• all UUP dump form fields are encoded;
• the active request core is wired to the fixed helper.

Release status

This is a second release-blocker hotfix discovered by the first real E2E run after PR #15 was merged. Keep this PR Draft until Windows self-hosted validation passes and the real ISO E2E is rerun successfully.

[Regstar2]

Superseded by consolidated Draft PR #23. The PowerShell 5.1 form-POST fix and regression coverage are included there so release hardening has one review/validation surface. This PR is being closed without merge.