Follow-ups on the #60987 salvage (review pass):
- _refresh_fallback_model: keep last known-good chain on transient
config.yaml read/parse failure (user mid-edit, torn write) — only a
successful read that lacks the key clears the chain. Previously a
refresh error wiped a cached agent's working fallback for the turn.
- Move the cached-agent refresh+apply OUTSIDE the agent-cache lock:
config.yaml read is disk I/O and the idle-sweep watcher contends on
that lock (same reasoning as #52197). Per-session turn serialization
keeps the post-lock apply safe.
- _apply_fallback_chain_to_agent: clear _unavailable_fallback_keys when
chain content actually changes, so an entry re-configured mid-uptime
(e.g. credentials added) is retried instead of staying suppressed for
the cached agent's lifetime; no-op refreshes keep the memo.
- Tests: cwd-independent source pin (Path(__file__) anchor), pin the
reuse-path apply call, + regression tests for last-known-good, memo
clear-on-change, memo keep-on-unchanged (mutation-verified).
Pin reload + cached-agent apply helpers for #60955 so a mid-uptime
fallback chain change reaches messaging sessions without a restart.
(cherry picked from commit fafb341035)
