[codex] Add DeepSeek V4 Flash to freebuff #6550

Job	Run time
build-and-check	3m 3s
test-integration-packages/internal	1m 2s
test-integration-packages/billing	1m 6s
test-integration-web	55s
test-integration-sdk	1m 20s
test-.agents	55s
test-common	43s
test-sdk	45s
test-integration-packages/agent-runtime	50s
test-cli	54s
test-web	3m 24s
test-packages/billing	39s
test-integration-common	40s
test-integration-cli	47s
test-packages/internal	58s
test-packages/agent-runtime	42s
	18m 43s

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