Support Pebble expressions in AI Copilot

[anna-geller]

Feature description

AI Copilot currently hallucinates Pebble expressions that do not exist (e.g. {{ inputs.selected_columns | jsonify }}).

The Copilot should be able to access the list of supported Pebble expressions (functions, filters, etc.) so it can generate valid syntax instead of inventing unsupported ones.