fix: update test assertions to match current behavior
- BuildTest: nix error changed from "No such file or directory" to "does not exist"
- PageControllerTest: Sign In is now a <.button> component, not raw CSS classes
- OrchestrationTest: process_deployment returns {ok, request_id} not a 3-tuple with task
Fixes #321, #322, #323
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>