Mark connection test as failed if sidecar returns validation error

The direct connection form allows users to test connecting to a Kafka cluster or Schema Registry before saving the connection. If the sidecar failed to connect to the resource, the direct connection form marks the connection as failed:

The sidecar validates all provided fields before attempting the connection. If fields violated the validation rules, it returns a client-side HTTP error. At the moment, the direct connection form renders a green check mark above the form if the sidecar returns a validation error:

It should show an error notification, similar to the first screenshot, instead.

Also, for OAuth credentials, we should validate the client secret in the direct connection form. I captured it separately: https://github.com/confluentinc/vscode/issues/2703