Update more agents to use gemini 3.1 flash #5850

Job	Run time
build-and-check	2m 55s
test-integration-packages/internal	1m 18s
test-integration-packages/billing	1m 33s
test-integration-web	41s
test-integration-packages/agent-runtime	40s
test-integration-cli	36s
test-integration-sdk	1m 22s
test-integration-common	1m 5s
test-packages/agent-runtime	53s
test-packages/billing	51s
test-common	41s
test-cli	56s
test-packages/internal	50s
test-.agents	1m 6s
test-sdk	43s
test-web	51s
	17m 1s

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