[codex] Hide empty subagent spawn errors #6693

Job	Run time
build-and-check	2m 49s
test-integration-packages/internal	1m 7s
test-integration-packages/billing	1m 6s
test-integration-sdk	1m 17s
test-integration-common	40s
test-cli	54s
test-integration-web	46s
test-integration-packages/agent-runtime	50s
test-packages/billing	43s
test-common	40s
test-integration-cli	40s
test-web	50s
test-packages/agent-runtime	52s
test-.agents	40s
test-packages/internal	52s
test-sdk	46s
	15m 32s

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