feat: cooking assistant can propose adding items to a shopping list

Second tool alongside createRecipe, same safety shape: addToShoppingList's
execute only validates/echoes input, no DB write. The proposal card lets
the user pick an existing list (fetched lazily) or name a new one, then
confirms through the exact two-call flow AddToShoppingListButton already
uses — POST /api/v1/shopping-lists (if new) then POST .../items — no new
persistence code path.

generateMealPlan intentionally not built as a tool: the existing
/api/v1/ai/meal-plan/generate endpoint generates and writes in one step
with a week/preferences input, not a specific plan payload, so it has no
"save this exact draft" entry point to confirm against without a larger
refactor. Documented as a known gap rather than forcing a weaker pattern.

v0.46.0

apps/web/lib/ai/tools/add-to-shopping-list-tool.ts

@@ -0,0 +1,27 @@
+import { tool } from "ai";
+import { z } from "zod";
+
+const AddToShoppingListInputSchema = z.object({
+  items: z.array(z.object({
+    rawName: z.string().min(1).max(200).describe("Ingredient/item name only, e.g. 'flour' — never combined with quantity/unit"),
+    quantity: z.number().optional().describe("A number only, e.g. 0.25, 1.5, 2"),
+    unit: z.string().max(50).optional().describe("e.g. 'cup', 'tbsp', 'g'"),
+  })).min(1).max(100),
+  suggestedListName: z.string().max(100).optional().describe("Suggested name if the user wants a new list, e.g. 'Weekend BBQ'"),
+});
+
+export type AddToShoppingListToolInput = z.infer<typeof AddToShoppingListInputSchema>;
+
+/**
+ * Lets the cooking assistant propose adding items to a shopping list — same
+ * safety shape as createRecipeTool: `execute` only validates/echoes the
+ * input, no DB write. The chat UI resolves which list (existing or new)
+ * and POSTs through the existing /api/v1/shopping-lists(/{id}/items)
+ * endpoints, the same ones AddToShoppingListButton already uses.
+ */
+export const addToShoppingListTool = tool({
+  description:
+    "Propose adding ingredients/items to a shopping list, e.g. from a recipe just discussed. This only drafts the item list for the user to review and pick a target list — it does not save anything. Only call this when the user has actually asked to add items to a shopping list.",
+  inputSchema: AddToShoppingListInputSchema,
+  execute: async (input: AddToShoppingListToolInput) => input,
+});