fix(f4): harden schema-columns enrichment + ValidationError guard + staged-helper note

harness/.pi/extensions/tht-gate.js

@@ -651,10 +651,19 @@ export default function (pi) {
 			const phase = phaseId(ctx, currentPhase(ctx, session));
 
 			// Enrich each table with its full catalog columns (deterministic source).
-			const enriched = tables.map((t) => {
-				const cat = JSON.parse(tht(ctx, ["schema", "columns", t.name, "--json"]));
+			const enriched = [];
+			for (const t of tables) {
+				let cat;
+				try {
+					cat = JSON.parse(tht(ctx, ["schema", "columns", t.name, "--json"]));
+				} catch (e) {
+					const msg = (e.stderr || e.message || String(e)).toString().trim();
+					return textResult(
+						`Tabella '${t.name}' non caricabile dal catalogo (${msg}). Proponi solo tabelle presenti nel catalogo (usa 'tht schema render' / 'tht search' per verificarne i nomi).`,
+					);
+				}
 				const suggested = new Set(t.suggested_columns ?? []);
-				return {
+				enriched.push({
 					id: t.id,
 					name: t.name,
 					kind: t.kind,
@@ -662,8 +671,8 @@ export default function (pi) {
 					description: cat.description ?? "",
 					rationale: t.rationale ?? "",
 					columns: cat.columns.map((c) => ({ ...c, suggested: suggested.has(c.name) })),
-				};
-			});
+				});
+			}
 
 			const widget = buildSchemaLinkingRequest({
 				id: `u${Date.now()}`,