fix(protocol): pin AdCP 3.1 stable #2015

Job	Run time
Downstream import smoke (representative consumer symbols)	1m 4s
Postgres conformance tests (Postgres 16)	1m 23s
AdCP storyboard runner — sales-proposal-mode (proposal_finalize)	1m 36s
AdCP storyboard runner — examples/multi_platform_seller (PlatformRouter)	1m 53s
Test Python 3.13	6m 11s
Test Python 3.12	6m 46s
AdCP storyboard runner — examples/seller_agent.py (@adcp/sdk adcp-3.1)	2m 43s
AdCP storyboard runner — v3 reference seller (translator)	2m 49s
Test Python 3.11	6m 50s
v3 reference seller — pytest (respx-mocked upstream)	52s
AdCP storyboard runner — examples/seller_agent.py (@adcp/sdk adcp-3.0)	2m 18s
Validate schemas are up-to-date	20m 55s
Test Python 3.10	6m 50s
Validate conventional commit format	0s
AdCP storyboard runner — examples/seller_agent.py	3s
	1h 2m 13s

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