move gemini-with-fallbacks into agent-runtime #3925

Job	Run time
build-and-check	2m 31s
test-integration-.agents	35s
test-integration-backend	35s
test-integration-cli	30s
test-integration-common	34s
test-integration-npm-app	38s
test-integration-packages/agent-runtime	33s
test-integration-web	32s
test-.agents	28s
test-npm-app	36s
test-web	32s
test-backend	2m 20s
test-common	32s
test-packages/agent-runtime	30s
test-cli	34s
	12m 0s

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