Fix virtualUsers conversion to string in K6BuilderExtensions

[eerhardt]

Aspire Args that are primitive values must be strings. The Core Aspire doesn't know how to format primitive values - ex. should a bool be False, false, or FALSE?

Closes dotnet/aspire#13423

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• New integration
  • Docs are written
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

Other information

[Copilot]

Pull request overview

This PR fixes a bug in the K6 integration where the virtualUsers parameter (an int) was being passed directly to WithArgs() instead of being converted to a string. According to Aspire's requirements, all primitive values must be explicitly converted to strings since the core framework doesn't handle formatting of primitives.

Key Changes

• Converted virtualUsers parameter to string using ToString(CultureInfo.InvariantCulture) for culture-independent formatting