better error message for agent when spawn tool as agent #5608

Job	Run time
build-and-check	3m 2s
test-integration-packages/billing	1m 3s
test-integration-sdk	40s
test-integration-cli	43s
test-integration-web	45s
test-common	40s
test-packages/internal	42s
test-integration-common	42s
test-integration-packages/internal	1m 5s
test-integration-packages/agent-runtime	46s
test-sdk	45s
test-cli	50s
test-web	51s
test-packages/agent-runtime	43s
test-.agents	36s
test-packages/billing	41s
	14m 34s

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