Discover form field names from recent submissions (resolver helper)

Sample recent submissions for a form and return the unique field names.

Use this to populate the form_fields[] array on a new_form_submitted trigger node config at create time, mirroring what NodeConfigPanel.handleFormSelect does on the frontend when the user selects a form in the dropdown.

Without these names pre-populated, downstream nodes cannot reference submitted values via ${<trigger_node_id>.<field>} until the user re-selects the form in the UI.

Calls forms handlers in-process (no voice round-trip). Forms data lives in ClickHouse via apps.forms() — sampled submissions are paginated server-side; we only ever read per_page=10 here.

Returns {form_id, fields: [str], sample_size: int}. Empty fields means no submissions exist yet — the caller should fall back to asking the user for the field list, or accept that ${trigger.<field>} references will only resolve once a real submission arrives.