Use DeepSeek agent for BuffBench #6503

Job	Run time
build-and-check	3m 3s
test-integration-packages/internal	1m 4s
test-integration-packages/billing	1m 7s
test-integration-packages/agent-runtime	40s
test-integration-web	42s
test-integration-sdk	1m 18s
test-integration-common	50s
test-integration-cli	50s
test-common	39s
test-packages/internal	49s
test-packages/billing	42s
test-.agents	46s
test-cli	56s
test-packages/agent-runtime	49s
test-web	1m 16s
test-sdk	43s
	16m 14s

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