Return handlebale error on denied outbound HTTP requests

[kate-goldenring]

@rylev found a bug in the shim using the new conformance tests suite. For the failing conformance test, we make a request to the app containing the url for it to make a downstream request to. If the app doesn't have permission, that call should fail, and the app will return a 500 with the body of the response set to the display of the ErrorCode encountered.

 // Get the incoming response.
        let response = match outgoing_handler::handle(outgoing_request, None) {
            Ok(r) => r,
            Err(e) => {
                return_response(outparam, 500, e.to_string().as_bytes());
                return;
            }
        };

The fix was added to Spin CLI in https://github.com/fermyon/spin/pull/2512/files#diff-d089e464d241060ca01fb6433e027427bf7c3248571995bc8d8b9712776f6166R726 which will be in release v2.6.0

Bumping to Spin v2.6 should fix the test

[adamreese]

Can this be closed with #144?