[codex] Restore Carbon ads fallback #6746

Job	Run time
build-and-check	2m 50s
test-integration-packages/internal	1m 5s
test-integration-packages/billing	1m 6s
test-integration-common	51s
test-integration-packages/agent-runtime	45s
test-integration-sdk	1m 26s
test-integration-cli	51s
test-integration-web	40s
test-.agents	41s
test-cli	1m 1s
test-packages/billing	51s
test-web	57s
test-packages/agent-runtime	41s
test-common	40s
test-packages/internal	47s
test-sdk	44s
	15m 56s

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