Managed subscriptions seller fails the SDK's own capability validation (principal fields, idempotency, supported_versions) and needs undocumented wiring over MCP #1771

Summary

A seller on the managed reporting-subscriptions path (ReliableReportingService(subscriptions=SubscriptionOptions(...)), adcp 10.0.0b4) advertises a get_adcp_capabilities response that the Python SDK's own client rejects. The client validates against the bundled 3.3.0-beta.3 schemas, and a Python buyer fails on the Python seller with:

get_adcp_capabilities response failed schema validation at /adcp/idempotency: 'idempotency' is a required property
  /adcp/principal/optimistic_concurrency 'optimistic_concurrency' is a required property
  /adcp/principal/max_reporting_destinations ... reporting_destination_offerings ... suspension_interval_seconds

ManagedSubscriptions.advertise_principal always adds the reporting_destinations section to adcp.principal.supported_sections. Under 3.3.0-beta.3 that section requires max_reporting_destinations, reporting_destination_offerings and suspension_interval_seconds, and principal itself requires optimistic_concurrency. The SDK fills none of these, and the seller guide doesn't say the host must. The service's supported_versions also defaults so that a client negotiates 3.0 or 3.1 and refuses get_principal ("get_principal is unavailable in AdCP 3.1") unless the host passes serve(supported_versions=("3.2", "3.3-beta.3")).

To get the interop seller working I had to:

• set adcp.idempotency, adcp.principal.{optimistic_concurrency, max_reporting_destinations, reporting_destination_offerings, suspension_interval_seconds} and experimental_features: ["account.requests"] in the handler's capabilities;
• pass supported_versions;
• pass context_factory=auth_context_factory; without it list_accounts gets no caller_identity over MCP;
• pass a caller_resolver; the default needs an AccountAwareToolContext, so get_reporting_status over MCP fails with "Reporting service configuration failed".

See tests/interop/matrix/py/seller.py in adcontextprotocol/adcp#8679.

Expected

The managed path either fills the principal fields it implies (or refuses to start without them, naming them), or the guide lists every host-supplied field. A seller built per docs/reporting-subscriptions-seller.md and served with serve() should pass the SDK's own response validation, and should serve get_reporting_status to bearer callers over MCP.

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