fix: await server function error handlers

[birkskyum]

Await serverFunctions.onError handlers before serializing errors. Async handlers resolving to undefined preserve the original error.

Includes regression tests and updated documentation.

• Fixes [Bug?]: an async serverFunctions.onError handler is not awaited #2268

cc @adipascu

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Latest commit: b608338

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commit: b608338

[adipascu]

Ran my production handler under the awaited contract, every path behaves the same and sync handlers are unaffected. Drop-in for me, thanks.

One edge worth a guard: the call sits inside catch (x), so a handler that throws or rejects (a failed await flush() is now the easy way there) escapes handleServerFunction, and the client gets an opaque 500 with no X-Error in place of the original error.

try {
  const replacement = await onServerFunctionError?.(thrown);
  return replacement ?? thrown;
} catch {
  return thrown;
}

A spec for a rejecting handler and one for an async handler resolving to a Response would pin these down.

[birkskyum]

@adipascu , good ideas, added the spec, and the reutrn of the original error.