[codex] Add DeepSeek V4 Flash to freebuff (#641) #6557

Job	Run time
build-and-check	2m 57s
test-integration-packages/internal	1m 4s
test-integration-packages/billing	1m 10s
test-integration-packages/agent-runtime	51s
test-integration-cli	53s
test-integration-sdk	1m 20s
test-packages/agent-runtime	47s
test-integration-common	47s
test-packages/billing	54s
test-cli	53s
test-.agents	39s
test-integration-web	43s
test-common	50s
test-packages/internal	49s
test-web	1m 15s
test-sdk	49s
	16m 41s

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