The optional 'provider' field on Adapter / AdapterInput / FromProtocolInput
existed as a registry filter: requests with a different model.provider could
not find adapters that set it. After the four-axis migration no adapter
needed it (and an earlier pass removed it from the five migrated providers
because setting it broke session/llm-native tests).
Drop the field entirely and collapse the registry to a single-tier protocol
lookup. If a future deployment genuinely needs to be scoped (e.g. an
Azure-only OpenAI Responses adapter), reintroduce as 'scopedTo' with an
explicit name. Solve when needed, not before.
Also drops the test that exercised the now-removed two-tier lookup
('prefers provider-specific adapters over protocol fallbacks').
Kit Langton
bb7f52b24d