[codex] Add DeepSeek V4 Flash to freebuff #6555

Job	Run time
build-and-check	2m 59s
test-integration-packages/internal	1m 10s
test-integration-packages/billing	1m 11s
test-integration-sdk	1m 23s
test-integration-web	54s
test-integration-cli	42s
test-packages/internal	49s
test-integration-packages/agent-runtime	40s
test-integration-common	38s
test-packages/agent-runtime	49s
test-common	41s
test-sdk	55s
test-.agents	51s
test-packages/billing	53s
test-web	1m 5s
test-cli	50s
	16m 30s

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