refactor: improve type safety patterns from @ts-expect-error cleanup

Amp-Thread-ID: https://ampcode.com/threads/T-019bb3bd-f607-735a-b1a8-fce5fe4f0125
Co-authored-by: Amp <amp@ampcode.com>

browser_tests/fixtures/utils/subgraphUtils.ts

@@ -0,0 +1,58 @@
+/**
+ * Represents a subgraph's graph object with inputs and outputs slots.
+ */
+export interface SubgraphGraph {
+  inputs: SubgraphSlot[]
+  outputs: SubgraphSlot[]
+}
+
+export interface SubgraphSlot {
+  label?: string
+  name?: string
+  displayName?: string
+}
+
+/**
+ * Type guard to check if a graph object is a subgraph.
+ */
+export function isSubgraph(graph: unknown): graph is SubgraphGraph {
+  return (
+    graph !== null &&
+    typeof graph === 'object' &&
+    'inputs' in graph &&
+    'outputs' in graph &&
+    Array.isArray((graph as SubgraphGraph).inputs) &&
+    Array.isArray((graph as SubgraphGraph).outputs)
+  )
+}
+
+/**
+ * Assertion function that throws if the graph is not a subgraph.
+ */
+export function assertSubgraph(
+  graph: unknown,
+  message = 'Not in subgraph'
+): asserts graph is SubgraphGraph {
+  if (!isSubgraph(graph)) {
+    throw new Error(message)
+  }
+}
+
+/**
+ * Inline assertion for use inside page.evaluate() browser context.
+ * Returns a string that can be used with Function constructor or eval.
+ */
+export const SUBGRAPH_ASSERT_INLINE = `
+  const assertSubgraph = (graph) => {
+    if (
+      graph === null ||
+      typeof graph !== 'object' ||
+      !('inputs' in graph) ||
+      !('outputs' in graph) ||
+      !Array.isArray(graph.inputs) ||
+      !Array.isArray(graph.outputs)
+    ) {
+      throw new Error('Not in subgraph');
+    }
+  };
+`