Revert "use async closure"

[Jaltaire]

I'm observing that this is causing problems using async-openai-wasm in Axum/Aide route handlers with async_openai_wasm::Client constructed inside a shared AppState:

error: implementation of `Send` is not general enough
  --> ...
   |
16 |             aide::axum::routing::post_with(POST::on_request, POST::on_docs),
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Send` is not general enough
   |
   = note: `Send` would have to be implemented for the type `&AppState<D>`
   = note: ...but `Send` is actually implemented for the type `&'0 AppState<D>`, for some specific lifetime `'0`

Reverting the change (not using async closures) precludes this issue.

[ifsheldon]

Hmmmm, interesting and weird. Can you open an issue and give me a reproducer? Any clue why this happens? I think these changes are only internal, never expect these to affect public APIs. I realize I didn't enable issues in my repo, sorry about that.

[ifsheldon]

Can you check if commit 1764a845e84e9685d64520b36b4b9d825be629a0 works? I've simplified the code a bit.