Add Gemini Pro freebuff model #6307

Job	Run time
build-and-check	2m 50s
test-integration-packages/billing	1m 16s
test-integration-packages/internal	1m 4s
test-integration-web	51s
test-integration-sdk	1m 20s
test-integration-common	45s
test-integration-cli	56s
test-packages/billing	46s
test-cli	1m 1s
test-packages/internal	44s
test-packages/agent-runtime	41s
test-integration-packages/agent-runtime	44s
test-sdk	42s
test-.agents	47s
test-common	49s
test-web	1m 3s
	16m 19s

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