Add optional schema for subagents' params to help models with tool calls #6061

Job	Run time
build-and-check	3m 9s
test-integration-packages/internal	1m 7s
test-integration-packages/billing	1m 3s
test-integration-common	45s
test-integration-packages/agent-runtime	41s
test-integration-web	46s
test-integration-sdk	1m 18s
test-sdk	55s
test-integration-cli	44s
test-common	45s
test-.agents	43s
test-packages/internal	48s
test-web	44s
test-cli	49s
test-packages/agent-runtime	50s
test-packages/billing	41s
	15m 48s

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