[codex] Restore Carbon ads fallback #818

Job	Run time
build-freebuff	1m 8s
build-and-smoke-freebuff-windows	2m 19s
e2e-startup	1m 8s
e2e-help-command	1m 20s
e2e-code-edit	56s
e2e-terminal-command	54s
e2e-ads-behavior	1m 9s
e2e-slash-commands	1m 10s
e2e-version	54s
e2e-agent-startup	2m 19s
	13m 17s

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