refactor(db): typed query helpers, shared test FormData helper

Replace raw db.execute tuple casts with queryRows/rowsFrom/execResult/
affectedRows helpers from lib/db, drop redundant mysql2 casts on typed
query builders, and centralize per-test fakeForm into test/fake-form.
Update db mocks in tests so helpers resolve against mocked execute.

src/actions/admin-articles.test.ts

@@ -37,6 +37,7 @@ vi.mock("next/navigation", () => ({
 	},
 }));
 vi.mock("@/lib/db", async () => {
+	const { createDbHelpers } = await import("@/test/db-helpers");
 	const { MySqlDialect } = await import("drizzle-orm/mysql-core");
 	const dialect = new MySqlDialect();
 	const database = {
@@ -85,7 +86,11 @@ vi.mock("@/lib/db", async () => {
 		transaction: async (fn: (tx: unknown) => unknown): Promise<unknown> =>
 			fn(database),
 	};
-	return { ...(await import("@/db/schema")), db: database };
+	return {
+		...(await import("@/db/schema")),
+		...createDbHelpers(database.execute),
+		db: database,
+	};
 });
 
 import { articleEditToken } from "@/lib/article-edit-token";