[codex] Hide empty subagent spawn errors (#713) #6694

Job	Run time
build-and-check	2m 29s
test-integration-packages/internal	1m 9s
test-integration-packages/billing	1m 7s
test-integration-web	55s
test-integration-common	42s
test-integration-packages/agent-runtime	54s
test-integration-cli	44s
test-integration-sdk	1m 19s
test-packages/agent-runtime	46s
test-packages/billing	42s
test-web	58s
test-sdk	40s
test-common	50s
test-.agents	43s
test-cli	51s
test-packages/internal	58s
	15m 47s

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