feat: add gemini-tester agent for testing Google Gemini CLI #5455

Job	Run time
build-and-check	2m 26s
test-cli	1m 15s
test-integration-common	41s
test-.agents	41s
test-packages/billing	43s
test-web	42s
test-packages/internal	53s
test-common	41s
test-packages/agent-runtime	53s
test-integration-packages/billing	41s
test-sdk	51s
test-integration-.agents	49s
test-integration-sdk	45s
test-integration-cli	40s
test-integration-web	40s
test-integration-packages/agent-runtime	38s
test-integration-packages/internal	41s
	14m 40s

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