fix(ext/http): route invalid async responses to onError

[subotac]

Fixes #36425.

Invalid values returned asynchronously from Deno.serve handlers were validated inside a promise fulfillment callback, after the rejection handler had already been attached to the original handler promise. The
resulting TypeError therefore bypassed onError.

Attach error handling after response conversion and fall back to the internal 500 response if an asynchronous onError result also fails. Extend the existing serve callback test to cover both synchronous and
asynchronously resolved invalid values.

Tests:

• cargo test -p unit_tests --test unit -- handleServeCallbackReturn
• cargo test -p unit_tests --test unit -- serve
• formatting and targeted JS/TS lint checks

[deno-cla-assistant]

Deno Individual Contributor License Agreement

The following contributors need to sign the CLA before this PR can be merged:

• @subotac

Click here to review and sign the CLA | Re-run CLA check

---

This is an automated message from CLA Assistant