Follow-up to #60168's salvage: aggregate_moa_context() is the third
independent MoA call path; assert its aggregator call receives the
custom-provider request_overrides.extra_body via **agg_runtime.
Slot_runtime resolved the provider's real API surface (including api_mode)
but only forwarded base_url and api_key to call_llm, dropping api_mode.
This caused Copilot GPT-5.x reference slots to hit /chat/completions
instead of the Responses API, returning 400 unsupported_api_for_model.
- _slot_runtime: forward api_mode from resolve_runtime_provider
- call_llm: accept explicit api_mode param, override task config
- 4 regression tests for propagation, omission, and signature
