engine: warn on WASM load failure when not crossOriginIsolated (#186252)

## Description

Fixes #142822.

When a Flutter WASM web app fails to load because the host page isn't
cross-origin isolated (no COOP/COEP headers), the underlying WebAssembly
errors are obscure and don't point at the actual problem. Examples users
currently see:

* `ReferenceError: SharedArrayBuffer is not defined`
* `RuntimeError: WebAssembly.instantiate(): shared memory not enabled`
* `TypeError: Failed to execute 'compileStreaming' on 'WebAssembly'`

This PR wraps the WASM entrypoint load in `_loadWasmEntrypoint` in a
try/catch. On failure, it checks `window.crossOriginIsolated === false`.
When that's the case, it emits a single `console.warn` that:

* Names the two missing HTTP headers (`Cross-Origin-Opener-Policy:
same-origin` and `Cross-Origin-Embedder-Policy: require-corp`).
* Suggests building with the JavaScript compile target instead of
`dart2wasm` if the developer can't change the server headers.
* Links to the canonical [web.dev COOP/COEP
guide](https://web.dev/articles/coop-coep).

The original error is re-thrown — the existing failure path is
unchanged. This is purely additive diagnostic output.

## Tests

No JS test infrastructure exists for
`engine/src/flutter/lib/web_ui/flutter_js/`. The change is also
diagnostic-only — it doesn't alter runtime behavior beyond the new
console.warn. Requesting a test exemption.

This PR replaces #186236 (closed for a clean conversation timeline).

## Pre-launch Checklist

- [x] I read the [Contributor Guide].
- [x] I read the [Tree Hygiene] page.
- [x] I read and followed the [Flutter Style Guide].
- [x] I signed the [CLA].
- [x] All existing tests are passing.

[Contributor Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/

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Co-authored-by: Harry Terkelsen <1961493+harryterkelsen@users.noreply.github.com>