[codex] Add DeepSeek V4 Flash to freebuff #626

Job	Run time
build-freebuff	1m 16s
build-and-smoke-freebuff-windows	2m 12s
e2e-slash-commands	58s
e2e-startup	1m 4s
e2e-code-edit	1m 5s
e2e-ads-behavior	1m 13s
e2e-help-command	1m 7s
e2e-agent-startup	1m 55s
e2e-terminal-command	1m 11s
e2e-version	1m 12s
	13m 13s

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