chore: update lint rules, add tests

web/eslint-plugins/__tests__/helpers.ts

@@ -0,0 +1,34 @@
+// Shared harness for testing the custom ESLint rules in eslint-plugins/.
+//
+// Two strategies, picked per rule in the test files:
+//   - RuleTester (eslint)  — pure string cases, no filesystem. Best fit for the
+//     design-tokens rules that only inspect the postcss AST of Svelte `<style>`.
+//   - Linter API + fixtures — needed when a rule reads real files. The isolation
+//     rule resolves imports via fs (targets must exist on disk) and
+//     no-undefined-in-svelte reads <cwd>/src/app.css for the token dictionary.
+//     `verifyInFixtures` runs a rule with cwd fixed to `__fixtures__/`, so both
+//     work without touching process.cwd() or the real src/ tree.
+import { Linter } from "eslint";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+const HERE = path.dirname(fileURLToPath(import.meta.url));
+
+export const FIXTURES_DIR = path.resolve(HERE, "../__fixtures__");
+export const FIXTURES_SRC = path.join(FIXTURES_DIR, "src");
+
+/** Flat-config shape the Linter.verify accepts. */
+export type FlatConfig = Parameters<Linter["verify"]>[1];
+
+/**
+ * Run a rule (or a config) against `code` as if it were the fixture file
+ * `relFile` under `__fixtures__/src`. Returns the lint messages.
+ */
+export function verifyInFixtures(
+	config: FlatConfig,
+	code: string,
+	relFile: string,
+) {
+	const linter = new Linter({ configType: "flat", cwd: FIXTURES_DIR });
+	return linter.verify(code, config, path.join(FIXTURES_SRC, relFile));
+}