fix: propagate BPMN render failures from browser to Node in diagram rendering

[Copilot]

The browser-side renderer was setting window.__diagramRendered = true in both success and catch paths, causing Puppeteer to take a blank screenshot on render failure and the CLI to report success.

Changes

• Browser (HTML template): Catch block now sets window.__diagramError with the error message instead of window.__diagramRendered = true
• Node/Puppeteer: waitForFunction resolves on either __diagramRendered (success) or __diagramError (failure); if the latter is set, throws back to the caller

// Browser: catch block now signals error instead of success
window.__diagramError = err instanceof Error ? err.message : String(err);

// Node: waitForFunction resolves on either outcome
await page.waitForFunction(
  'window.__diagramRendered === true || window.__diagramError !== undefined',
  { timeout: 15000 },
);

// Node: propagate render error
const renderError = await page.evaluate('window.__diagramError');
if (renderError) {
  throw new Error(`BPMN diagram render failed: ${renderError}`);
}

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.