Use DeepSeek agent for BuffBench #6504

Job	Run time
build-and-check	3m 6s
test-integration-packages/billing	1m 13s
test-integration-packages/internal	1m 6s
test-integration-sdk	1m 24s
test-integration-common	46s
test-integration-web	56s
test-integration-packages/agent-runtime	48s
test-integration-cli	45s
test-packages/agent-runtime	45s
test-cli	54s
test-packages/internal	53s
test-sdk	47s
test-.agents	43s
test-packages/billing	45s
test-common	43s
test-web	1m 12s
	16m 46s

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