fix(functions): keep edge function failures as FunctionsHttpError

[tushardev-365]

What

_request reads the failure body inside its except HTTPError handler:

raise FunctionsHttpError(
    response.json().get("error")
    or f"An error occurred while requesting your edge function at {exc.request.url!r}.",
    status_code,
) from exc

A failing edge function does not always answer with a JSON object. A 502/504 from the gateway returns HTML, and a body can be a valid JSON array or string. So:

non-JSON body   -> json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
JSON list body  -> AttributeError: 'list' object has no attribute 'get'

The caller gets that raw exception instead of FunctionsHttpError, so except FunctionsHttpError around invoke() does not catch the failure, and the status code and body are lost.

There is a second effect worth calling out: the or "An error occurred ..." fallback was unreachable in exactly the situation it was written for. It could only be reached once .json() had already succeeded and returned a dict — i.e. never for the malformed bodies it was meant to cover.

The relay path had the identical call:

raise FunctionsRelayError(response.json().get("error"))

Fix

Read the body through a small get_error_message helper in utils.py that returns None when the body is not JSON or not an object, so the existing fallback message does its job. Applied to both sites.

Tests

Two cases per flavour (async + sync): a non-JSON error body and a JSON body that is not an object. Both assert FunctionsHttpError. Reverting only the client call sites makes all four fail with JSONDecodeError / AttributeError.

Full functions suite: 69 passed, 1 skipped. _sync is generated, so the change was made in _async and regenerated with make build-sync.