[codex] Restore Carbon ads fallback (#741) #819

Job	Run time
build-and-smoke-freebuff-windows	2m 28s
build-freebuff	1m 19s
e2e-slash-commands	1m 16s
e2e-help-command	1m 5s
e2e-startup	1m 5s
e2e-version	55s
e2e-code-edit	1m 10s
e2e-ads-behavior	1m 11s
e2e-agent-startup	2m 15s
e2e-terminal-command	57s
	13m 41s

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