fix(cli): never print "[object Object]" from validate/inspect errors (#1810)

* fix(cli): use normalizeErrorMessage so validate/inspect never print "[object Object]"

The validate and inspect (layout) commands formatted thrown values with
`err instanceof Error ? err.message : String(err)`. When a browser/CDP/
Puppeteer protocol error or a structured page error reaches the formatter
as a plain object without a string `message`, `String(obj)` yields the
useless literal "[object Object]", hiding the real cause.

Route those paths through the existing shared `normalizeErrorMessage`
helper, which returns an Error's message, a string as-is, an object's
`.message` when present, or a compact JSON serialization otherwise (with
a key-list and String fallback for circular/opaque objects). Also fold
the duplicated local `errorMessage` helpers in batchRender and preview
into the same shared helper.

Covered by added assertions in errorMessage.test.ts for the no-message
object and Puppeteer-style protocol-error object cases.

* fix(cli): route remaining browser/process error sites through normalizeErrorMessage

The validate/inspect fix routed only those two commands through the shared
normalizeErrorMessage helper. The same err instanceof Error ? err.message :
String(err) pattern survived in the other commands that drive a headless
browser or an external process (ffmpeg, Docker, CDP) or surface a network
API error, so a thrown structured object without a string message would
still render as the useless literal [object Object].

Route those sites through the shared helper:
  snapshot.ts (the closest sibling to validate/inspect, same bug class),
  render.ts (Chrome launch + Docker build), capture/index.ts and
  commands/capture.ts (page-driven extraction), auth/browser.ts,
  browser/manager.ts (Puppeteer browser resolution), and the cloud/lambda
  paths (cloud/render.ts, cloudrun.ts, lambda/render-batch.ts,
  lambda/policies.ts, cloud/detectAspectRatio.ts) that surface API/network
  error objects.

Only the message-deriving expression changes; control flow and error
propagation are untouched. capture/index.ts keeps appending the stack for
real Errors and only routes the non-Error branch. Adds a helper test for a
structured CDP-style error object (code + nested data, no message).

packages/cli/src/commands/batchRender.ts

@@ -2,6 +2,7 @@ import { mkdirSync, readFileSync, writeFileSync } from "node:fs";
 import { dirname, join, resolve, sep } from "node:path";
 import { c } from "../ui/colors.js";
 import { errorBox } from "../ui/format.js";
+import { normalizeErrorMessage as errorMessage } from "../utils/errorMessage.js";
 import {
   loadProjectVariableSchema,
   reportVariableIssues,
@@ -90,10 +91,7 @@ function parseJson(raw: string, source: string): unknown {
   try {
     return JSON.parse(raw);
   } catch (error: unknown) {
-    throw new BatchRenderInputError(
-      "Invalid JSON in --batch",
-      `${source}: ${error instanceof Error ? error.message : String(error)}`,
-    );
+    throw new BatchRenderInputError("Invalid JSON in --batch", `${source}: ${errorMessage(error)}`);
   }
 }
 
@@ -239,7 +237,7 @@ export function prepareBatchRender(options: PrepareBatchRenderOptions): Prepared
   } catch (error: unknown) {
     throw new BatchRenderInputError(
       "Could not read --batch",
-      `${batchPath}: ${error instanceof Error ? error.message : String(error)}`,
+      `${batchPath}: ${errorMessage(error)}`,
     );
   }
 
@@ -310,10 +308,6 @@ function emitJsonEvent(event: Record<string, unknown>, json: boolean): void {
   if (json) console.log(JSON.stringify(event));
 }
 
-function errorMessage(error: unknown): string {
-  return error instanceof Error ? error.message : String(error);
-}
-
 async function renderBatchRow(
   row: PreparedBatchRow,
   manifest: BatchManifest,