micro-fix: resolve NativeCommandError in uv sync and ParserError in dynamic env var assignment

[GovindhKishore]

Description

This PR resolves critical issues in the Windows onboarding script (quickstart.ps1) that prevented users from completing the setup. It addresses both a syntax-level ParserError and a runtime NativeCommandError. The script crashed on Windows PowerShell 5.1 due to multi-argument Join-Path calls, which triggered PositionalParameterNotFound errors because that version only supports joining two path segments at a time.

Changes Made

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #4434

Changes Made

• Removed the invalid syntax $env:$SelectedEnvVar = $apiKey on line 504.
• Ensured session-level environment variable assignment is handled by the existing Set-Item cmdlet on line 505.
• Fixed the PowerShell ParserError that blocked script execution upon startup.
• Wrapped the command to temporarily use ErrorActionPreference = "Continue", preventing PowerShell from crashing when it sees uv status messages in the error stream.
• Added | Out-String to the sync command to safely store logs in $syncOutput without triggering a NativeCommandError.
• Implemented an explicit exit code check to ensure the script only stops for real installation failures, ignoring "false alarms" caused by stream redirection.
• Replaced all multi-argument Join-Path calls with [System.IO.Path]::Combine to eliminate 'PositionalParameterNotFound' errors and ensure full compatibility with all Windows PowerShell versions.

Testing

Describe the tests you ran to verify your changes:

• Unit tests pass (cd core && pytest tests/)
• Lint passes (cd core && ruff check .)
• Manual testing performed: Verified that the setup script now parses correctly and the hive welcome page is reached correctly.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• [] I have commented my code, particularly in hard-to-understand areas
• [] I have made corresponding changes to the documentation
• My changes generate no new warnings
• [] I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Screenshots (if applicable)

N/A