[DOM] Register error event when hydrating <embed> - #37403

Summary

onError on a hydrated <embed> never fires. The same element rendered with createRoot fires it, and onLoad works in both modes.

Root cause

error and load don't bubble, so React only receives them through a per-element listenToNonDelegatedEvent call. #26501 made setInitialProperties register both error and load for <embed> (grouped with source/link, ReactDOMComponent.js:1417), but hydrateProperties still had the pre-#26501 grouping with iframe/object, which registers only load (ReactDOMComponent.js:3168). So a hydrated <embed> has no error listener and onError (and any ancestor's emulated-bubble onError) never dispatches.

Fix

Move case 'embed' in hydrateProperties into the group that registers both error and load, matching the mount path.

Not changed here: the mount path also registers load for <source> while hydration registers only error. That difference dates to the same refactor; it can be aligned the same way if desired, but it's a separate surface so I left it out of this diff.

How did you test this change?

Added a test that server-renders an <embed> with onLoad/onError, hydrates it, and dispatches both events. On main it fails:

expect(handleError).toHaveBeenCalledTimes(1)
Expected number of calls: 1
Received number of calls: 0

With the fix it passes. Also ran:

• yarn test packages/react-dom/src/__tests__/ReactDOMEventListener-test.js — 24 passed (also with --prod and --release-channel=www-classic)
• yarn test on ReactDOMEventPropagation, ReactServerRenderingHydration, ReactDOMHydrationDiff, ReactBrowserEventEmitter, ReactDOMServerIntegrationElements, ReactDOMServerPartialHydration — all passing except one pre-existing [GATED, SHOULD FAIL] failure in ReactDOMServerPartialHydration that reproduces identically on main
• yarn flow dom-node, yarn linc, yarn prettier-check — clean