Fix Windows PATH issue in install.bat

[dscpinheiro]

I was trying to install the Smithy CLI on my Windows laptop earlier today, but ran into an issue since setx (which the previous script used) reads from the session %PATH% (system + user combined) and silently truncates at 1024 characters.

So I replaced that with PowerShell's SetEnvironmentVariable, which reads and writes only the user PATH from the registry without a length limit. Also added some extra validation such as preventing duplicates on re-installs and checking the exit code in case updating the path fails.

This shouldn't impact the Scoop installation method since it manages updating the PATH separately (via https://github.com/smithy-lang/scoop-bucket/blob/main/bucket/smithy-cli.json#L9).

Testing

• How did you test these changes?
  Followed the manual steps for Windows (from https://smithy.io/2.0/guides/smithy-cli/cli_installation.html) after replacing the install.bat file with the content from my fork. On my Workspace, confirmed the path was appended correctly and I could invoke smithy --version.

Before:

PS> [Environment]::GetEnvironmentVariable('PATH', 'User')
D:\Users\dspin\AppData\Local\Microsoft\WindowsApps; 

After:

PS> [Environment]::GetEnvironmentVariable('PATH', 'User')
D:\Users\dspin\AppData\Local\Microsoft\WindowsApps;D:\Program Files\Smithy
PS> smithy --version
1.70.0

Links

N/A

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[github-actions]

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[Copilot]

Pull request overview

Updates the Windows install.bat flow for Smithy CLI to avoid setx PATH truncation by switching to a PowerShell-based user PATH update, aiming to make installs more reliable on systems with long PATH values.

Changes:

• Replace setx with PowerShell [Environment]::SetEnvironmentVariable to update the user PATH without the setx 1024-character truncation behavior.
• Add basic validation to avoid appending duplicate install entries to the user PATH.
• Add an %ERRORLEVEL% check to stop and instruct the user if the PATH update fails.

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