[codex] Add DeepSeek V4 Flash to freebuff #6554

Job	Run time
build-and-check	3m 12s
test-integration-packages/internal	1m 17s
test-integration-packages/billing	1m 0s
test-integration-common	39s
test-integration-packages/agent-runtime	40s
test-integration-sdk	1m 17s
test-packages/billing	43s
test-integration-cli	39s
test-packages/internal	50s
test-common	40s
test-cli	1m 6s
test-.agents	40s
test-packages/agent-runtime	54s
test-sdk	58s
test-integration-web	42s
test-web	1m 8s
	16m 25s

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