feat(server): TenantRegistry.as_platform() adapter for serve() integration #1399

Job	Run time
Validate conventional commit format	7s
AdCP storyboard runner — v3 reference seller (translator)	1m 13s
Test Python 3.13	3m 37s
Test Python 3.10	3m 23s
Validate schemas are up-to-date	3m 49s
AdCP storyboard runner — examples/multi_platform_seller (PlatformRouter)	1m 42s
AdCP storyboard runner — examples/seller_agent.py	1m 35s
Test Python 3.12	3m 43s
AdCP storyboard runner — sales-proposal-mode (proposal_finalize)	1m 3s
v3 reference seller — pytest (respx-mocked upstream)	41s
Postgres conformance tests (Postgres 16)	1m 6s
Downstream import smoke (representative consumer symbols)	24s
Test Python 3.11	3m 17s
	25m 40s

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