Stop hiding default Python kernelspecs in kernel picker

[Copilot]

This change removes the logic that was hiding default Python kernelspecs (like python3, python, Python 3 (ipykernel)) from the kernel picker.

Problem:
Users were filing issues because they couldn't find kernelspecs they expected to see, leading them to believe the Jupyter extension wasn't working properly. The extension was automatically hiding default Python kernelspecs when:

• The kernelspec was for Python language
• It had no custom environment variables
• It matched default naming patterns

Solution:
The original rationale for hiding these kernelspecs was to avoid user confusion between Python environments and kernelspecs. However, this reasoning no longer applies since the kernel picker now has separate sections for "Python Environments" and "Jupyter Kernels", making the distinction clear to users.

Changes:

• Removed the conditional hiding logic in InterpreterSpecificKernelSpecsFinder.listKernelSpecsImpl()
• Added explanatory comment referencing this issue
• All discovered kernelspecs are now made visible to users

Impact:
Default Python kernelspecs will now appear in the kernel picker alongside custom kernelspecs, improving discoverability and reducing user confusion about missing kernels.

Fixes #15557.

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