Follow-up to the previous commit (which fixed the default/fallback
provider path). A mid-session `/model` switch stores a per-session
override bundle in `_session_model_overrides` that omitted
`request_overrides`, and the two consumers
(`_resolve_session_agent_runtime` fast path and
`_apply_session_model_override`) only copied
provider/api_key/base_url/api_mode. So switching *to* a custom provider
via `/model` did not apply its `extra_body`.
- `ModelSwitchResult` gains a `request_overrides` field, derived for the
switched provider via `_get_named_custom_provider` /
`_custom_provider_request_overrides` (the same overrides
`resolve_runtime_provider` surfaces for the default path).
- Both `/model` override-storage sites in slash_commands.py persist it.
- Both consumers apply it; `_apply_session_model_override` also clears a
stale value when switching to a provider that has none.
Extends tests/gateway/test_turn_request_overrides.py (3 new cases).
5d238be2ca