[codex] Restore Carbon ads fallback (#741) #6747

Job	Run time
build-and-check	3m 7s
test-integration-packages/billing	1m 4s
test-integration-packages/internal	1m 5s
test-integration-common	40s
test-integration-sdk	1m 25s
test-integration-packages/agent-runtime	51s
test-integration-web	44s
test-integration-cli	44s
test-common	47s
test-packages/billing	46s
test-packages/agent-runtime	56s
test-cli	58s
test-web	1m 3s
test-.agents	40s
test-packages/internal	52s
test-sdk	53s
	16m 35s

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