DecisioningCapabilities cannot publish ext / extensions_supported on get_adcp_capabilities #1844

Summary

get_adcp_capabilities allows a top-level ext object and extensions_supported in AdCP 3.x, but DecisioningCapabilities has no field for either, so a seller built on DecisioningPlatform cannot publish an extension on its capabilities response. The only way to add one today is to wrap PlatformHandler.get_adcp_capabilities and edit the response dict after the SDK projects it.

Minimal repro

adcp==9.5.1 (same on 9.4.0):

import dataclasses
from adcp.decisioning import DecisioningCapabilities

print("ext" in {f.name for f in dataclasses.fields(DecisioningCapabilities)})  # False
DecisioningCapabilities(specialisms=["sales-non-guaranteed"], ext={"acme": {"flag": True}})
# TypeError: DecisioningCapabilities.__init__() got an unexpected keyword argument 'ext'

Neither DecisioningPlatform.capabilities nor the request-scoped get_adcp_capabilities_for_request() (which returns a DecisioningCapabilities) can carry it, and PlatformHandler.get_adcp_capabilities projects each declared block into the response (handler.py, the caps.signals ... caps.compliance_testing projection, then experimental_features) with nothing for ext or extensions_supported.

Why it matters

The AdCP capabilities schema (3.2 get-adcp-capabilities-response.json) defines both ext and extensions_supported ("Extension namespaces this agent supports. Buyers can expect meaningful data in ext.{namespace} fields on responses from this agent"), and the SDK's client already reads them (supports("ext:scope3")). A seller has no supported way to write them.

Our case: the Scope3 embedded sales agent declares whether it accepts a create_media_buy with no creatives, which AdCP cannot express yet (proposal: adcontextprotocol/adcp#8723). It carries the answer as ext.scope3.create_without_creatives and has to wrap the handler to do so.

Suggested fix

Add two optional fields to DecisioningCapabilities, projected as they are:

ext: dict[str, Any] | None = None
extensions_supported: list[str] | None = None

and, in PlatformHandler.get_adcp_capabilities, copy them onto the response when set (validating extensions_supported against the schema pattern ^[a-z][a-z0-9_]*$, unique). Merging ext from get_adcp_capabilities_for_request() over the platform-level value would let a request-scoped platform vary it per tenant, as it can for the other blocks.

We will drop our handler wrapper when this ships.