[codex] Add Carbon fallback for CLI ads #6234

Job	Run time
build-and-check	3m 8s
test-integration-packages/billing	1m 2s
test-integration-packages/internal	1m 7s
test-integration-cli	38s
test-integration-sdk	1m 14s
test-cli	1m 2s
test-packages/internal	45s
test-integration-web	40s
test-packages/agent-runtime	42s
test-integration-common	49s
test-packages/billing	50s
test-.agents	40s
test-web	1m 9s
test-sdk	47s
test-common	39s
test-integration-packages/agent-runtime	39s
	15m 51s

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