[Mwrnk]

AsyncGeneratorWrapper.runDelegateFrame_ calls .then() directly on whatever the delegate's next, throw or return returns. An async iterator may return a plain {value, done} object, and ECMA-262 Yield* awaits innerResult, so native code accepts it, but with --language_out ECMASCRIPT_2017 or lower the outer generator rejects with this.delegate_[a.method](...).then is not a function.

Wrap the result in Promise.resolve(), as the throw-without-throw fallback a few lines below already does for delegate['return'](). Promises and thenables resolve as before, and a synchronous throw from the delegate still reaches the existing catch.

The runtime library's JS tests are not in the GitHub tree, so I tested end to end: built //:compiler_uberjar_deploy.jar from master and from this branch, compiled a script that drives yield* over plain-object next(), throw() and return() results (plus a thenable and an async generator as controls), and ran it in Node 22. On master all seven plain-object cases reject; with this change they all resolve, six of them exactly as native Node does, and the controls are unchanged. The seventh, return(v) during yield*, now finishes with v instead of the delegate's return value; master already does that with Promise-returning delegates, so I left it out of this change. bazelisk test //:all passes (435 tests). If there is a place for a runtime test I can add one there.

Fixes #4385