test(responses): add Splunk telemetry tests for responses endpoint

Add 9 tests covering _queue_responses_splunk_event helper and all six
telemetry paths (shield blocked, error, success for both streaming and
non-streaming). Include a no-op test for when background_tasks is None
and a test verifying no double-fire on blocked streaming requests.

Signed-off-by: Major Hayden <major@redhat.com>

Author: major