fix: preserve bare defaultInterpreterPath commands

[jibin jose (jibin7jose)]

Fixes #1917

Fix workspace-scoped python.defaultInterpreterPath values that use a bare command name, such as python3.12. When the command is unavailable on the extension-host PATH, keep it unchanged for native resolution instead of converting it to a workspace-relative path.

[Eleanor Boyd (eleanorjboyd)]

For separator-free values, could we use this precedence?

1. Found on PATH → return that executable, preserving existing behavior.
2. Names an existing directory in the workspace → return its absolute path so PET can find Python inside it.
3. Otherwise → pass the bare name unchanged to PET.

This would preserve workspace-relative environment folders such as .venv or env, which the new early return also treats as bare commands, while still fixing command names such as python3.12 that are missing from the extension host PATH.