fix(reactive_scripts_editor): make Python embedding opt-in to unblock Linux marketplace install

[pabloinigoblasco]

Summary

The v1.4.2 marketplace artifact of toolbox_reactive_scripts_editor carried a NEEDED dependency on the libpython present on the CI runner (Ubuntu 22.04 ships libpython3.10), so the plugin failed to dlopen on user machines without that exact runtime installed.

The underlying discovery bug — pybind11's bundled FindPythonLibsNew hijacks the Python lookup and ignores the dependency manager's cpython package — is documented separately and not addressed here. This PR is the pragmatic unblocker so Linux users can install the plugin from the marketplace today.

Approach

Make Python embedding compile-time optional.

• New CMake option PJ_REACTIVE_ENABLE_PYTHON, defaults to OFF.
• With Python disabled the build links no libpython, ships no Python stdlib, and the dialog renders the Python radio as disabled so Lua remains the only callable backend.
• Set PJ_REACTIVE_ENABLE_PYTHON=ON for local developer builds where Python is known to be available.

UI behaviour with Python disabled

• Language radio: Python disabled, Lua remains active.
• onToggled: rejects late radio_python checked events (e.g. from a restored config) so language_ stays on lua.
• onTick validation: returns a Lua-only message when language_ is python.
• executeScript dispatch: returns an explicit "Python not available in this build" error instead of crashing.

Files

File	Change
toolbox_reactive_scripts_editor/CMakeLists.txt	Gate libpython linking and Python stdlib install behind PJ_REACTIVE_ENABLE_PYTHON (default OFF).
toolbox_reactive_scripts_editor/reactive_script_editor.cpp	Disable Python radio in dialog, reject late checked events, narrow validation/execute paths to Lua when Python is unavailable at build time.
toolbox_reactive_scripts_editor/manifest.json	Version 1.4.2 → 1.4.1 — the Python-disabled build is functionally equivalent to the pre-Python release.

Total: +111 / -54 across 3 files.

Test plan

• Build with default options (no PJ_REACTIVE_ENABLE_PYTHON): plugin links cleanly, ldd shows no libpython NEEDED entry, dlopen succeeds on a clean Ubuntu without libpython3.10 installed.
• Build with PJ_REACTIVE_ENABLE_PYTHON=ON: Python radio active, Python scripts run as before.
• Dialog: Python radio is disabled (greyed out) in default build; clicking it does not toggle language_.
• Restored config with language=python: dialog falls back to Lua, no crash on tick or execute.