feat(testing): build_test_client(platform) async context manager #1229

Job	Run time
AdCP storyboard runner — examples/multi_platform_seller (PlatformRouter)	1m 30s
Downstream import smoke (representative consumer symbols)	28s
v3 reference seller — pytest (respx-mocked upstream)	42s
AdCP storyboard runner — examples/seller_agent.py	1m 28s
Validate conventional commit format	8s
AdCP storyboard runner — v3 reference seller (translator)	1m 27s
Test Python 3.11	3m 23s
Validate schemas are up-to-date	3m 37s
Postgres conformance tests (Postgres 16)	1m 6s
Test Python 3.10	3m 30s
Test Python 3.12	3m 35s
Test Python 3.13	3m 26s
	24m 20s

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