Fixes: #9198
When observation is enabled on the `MessageChannel`, the message to send is converted to a `MutableMessage`.
In case of `ErrorMessage` this causes a loss of `originalMessage` and may lead to `ClassCastException`
in the target error handler.
* Check for `ErrorMessage` in the `AbstractMessageChannel.sendWithObservation()` and create a new one
as a copy of original request, but including observation headers before performing `sendInternal()`
* Modify `IntegrationObservabilityZipkinTests` to verify an observation with an `ErrorMessage` and its handler.
**Auto-cherry-pick to `6.3.x` & `6.2.x`**
b0cbacfafb