PR Summary

Fixes #28274.

When Get-Help finds the help of a cmdlet or function in a file, CommandHelpProvider sets the HelpFile property of the command to the full path of that file. From then on, FindHelpFile()/GetHelpFile() load that path directly instead of searching for the help.

Reset() runs after Update-Help and when the UI culture changes. It clears the cached help, but leaves those full paths in place, so the session keeps loading the help from the same file:

• After Update-Help -Scope CurrentUser, help in $PSHOME that was loaded before keeps winning over the newly installed CurrentUser help, which is searched first otherwise (Get-Help keeps returning help it already loaded after Update-Help -Scope CurrentUser, which makes HelpSystem.Tests.ps1 flaky when $PSHOME has help #28274).
• After the UI culture changes, the help stays in the previous culture.

This PR makes CommandHelpProvider keep the original HelpFile value of every command it changes, and Reset() restores those values, so the help is searched for again. The original values are kept in a ConditionalWeakTable, so commands of removed modules aren't kept alive until the next reset. HelpFile still shows the full path of the help file after Get-Help, as before.

The new test in HelpSystem.Tests.ps1 covers the UI culture case, as it doesn't need help in $PSHOME: a script module with help in en-US and de-DE, Get-Help in en-US, then in de-DE. Without the fix, the second Get-Help returns the en-US help.

PR Context

Get-Help in a session returns outdated help after Update-Help or a change of the UI culture, until a new session is started. This also makes HelpSystem.Tests.ps1 fail when $PSHOME contains help: "Validate that get-help works for CurrentUserScope" installs the test help in the CurrentUser scope, but gets the help from $PSHOME that an earlier test loaded.

I built and tested the change on 7.6.9 (HelpSystem.Tests.ps1 passes, including with help in $PSHOME, and the new test fails without the fix). It applies cleanly to master, but I don't have the .NET 11 SDK master needs here, so the build on master is left to CI.

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  • Make sure you've added a new test if existing tests do not effectively test the code changed

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