After /model switches the model (both picker and text paths), the cached
agent's config signature becomes stale — the agent was updated in-place
via switch_model() but the cache tuple's signature was never refreshed.
The next turn *should* detect the signature mismatch and create a fresh
agent, but this relies on the new model's signature differing from the
old one in _agent_config_signature().
Evicting the cached agent explicitly after storing the session override
is more defensive — the next turn is guaranteed to create a fresh agent
from the override without depending on signature mismatch detection.
Also adds debug logging at three key decision points so we can trace
exactly what happens when /model + /retry interact:
- _resolve_session_agent_runtime: which override path is taken (fast
with api_key vs fallback), or why no override was found
- _run_agent.run_sync: final resolved model/provider before agent
creation
Reported: /model switch to xiaomi/mimo-v2-pro followed by /retry still
used the old model (glm-5.1).
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