Route assertions, paging clamp, 404/422/400 paths and mediator interaction
for the new PlayoutController/TroubleshootController endpoints; handler tests
for ValidateSequentialSchedule (valid/invalid/throwing) and
GetPlayoutHistoryDetails (found/not-found/malformed JSON via in-memory SQLite).
Refs #145#158
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
