Port the PR #52432 regression (fast turn then normal turn on a cached
gateway agent) onto the current _run_agent harness: the original test's
host file context no longer exists on main after the TurnRunner
extraction, so the scenario is re-expressed with the existing
_CapturingAgent fixture. Asserts init-time custom-provider extra_body
survives both a /fast turn (service_tier layered on top) and the
following normal turn (only the stale fast-mode key drops).
Salvaged-from: #52432
Co-authored-by: Heng Cai <abtion@outlook.com>
Carry provider-derived request_overrides through runtime resolution,
fallback projection, session /model state, restart rehydration, and
turn-route merge so named custom providers keep extra_body and related
overrides.
Teknium