set_hermes_default_model() calls reload_config() which resyncs _cfg_mtime,
so the mtime check inside get_available_models() never fires and the POST
response returns the stale cached default. Explicitly drop the TTL cache
after reload so the next read recomputes. Fixes the CI failure in
test_default_model_updates_hermes_config which the prior teardown-only
fix in this PR did not actually address.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>