_call_fallback_candidate_sync replans messages/tools for each resolved
destination, but its async mirror still shipped the caller's decorated
sections verbatim — the primary destination's markers (including a
direct-native tool marker) leaked to fallback candidates with different
cache contracts, and the relay saw the display label instead of the
resolved provider/api_mode. Mirror the sync path: resolve the
destination, replan both sections, thread provider/api_mode through
_relay_async_completion, and replan again for the auth-refresh retry
client. Mutation-checked: the new parity test fails on the verbatim
pass-through shape.
Follow-up to #76032 (#20880).
536754919d