Add fallback model #6237

Job	Run time
build-and-check	3m 10s
test-integration-packages/billing	1m 7s
test-integration-sdk	1m 15s
test-integration-packages/agent-runtime	50s
test-integration-packages/internal	1m 6s
test-integration-web	43s
test-integration-cli	39s
test-integration-common	45s
test-packages/agent-runtime	42s
test-common	42s
test-packages/billing	50s
test-.agents	49s
test-sdk	55s
test-cli	59s
test-packages/internal	52s
test-web	59s
	16m 23s

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