Assert boot screen renders, not just absence of fatals #6375

Job	Run time
build-and-check	3m 8s
test-integration-packages/internal	1m 19s
test-integration-packages/billing	1m 4s
test-integration-sdk	1m 17s
test-.agents	43s
test-integration-cli	52s
test-packages/agent-runtime	40s
test-integration-web	51s
test-integration-packages/agent-runtime	46s
test-integration-common	43s
test-sdk	53s
test-packages/internal	49s
test-common	52s
test-web	1m 18s
test-packages/billing	41s
test-cli	1m 3s
	16m 59s

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