Refine Start-PythonOnboarding.ps1: add -PrepareOnly and safer custom launch

[yourlastnamesoundslikeatypeofpasta]

Motivation

• Provide a way to prepare a Python project environment (venv + requirements) without automatically starting the server so users can opt out of immediate launch.
• Make custom launch mode safer by avoiding Invoke-Expression and running the intended executable directly with argument passing.
• Clarify the intended PowerShell/Python boundary where PowerShell orchestrates UX and lifecycle steps while Python is invoked only for Python-specific tasks.

Description

• Added a -PrepareOnly switch to scripts/Start-PythonOnboarding.ps1 that stops after creating the venv and installing requirements and prints an activation hint.
• Reworked custom launch mode to map python|python3|py to the venv interpreter and execute commands using the call operator (&) with @ServerArguments instead of Invoke-Expression.
• Preserved existing script (& $venvPython $ServerTarget) and module (& $venvPython -m $ServerTarget) launch behaviors.
• Updated scripts/README.md to add an entry for Start-PythonOnboarding.ps1.

Testing

• Performed static inspection and line-by-line review of the modified Start-PythonOnboarding.ps1 file which passed review.
• Verified repository operations with git commit and git show --stat --oneline which succeeded.
• Attempted to run pwsh -NoLogo -NoProfile -Command '$PSVersionTable.PSVersion' but the runtime is not available in this environment, so no end-to-end execution or Pester tests were run.

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Codex Task