## Summary
Follow-up to #751/#752. Code review identified a case where `_normalize_session_model_in_place` could call `session.save()` (which triggers a full session index rebuild) for sessions with `model: null` or missing model field.
Root cause: `_resolve_compatible_session_model(None)` returns `(default_model, True)` when a default exists — which was interpreted as "changed, needs save." But there's nothing to correct for a session with no model; the default is just a fallback for display purposes, not a cross-provider correction worth persisting.
Fix: capture `original_model` before calling `_resolve_compatible_session_model`. Only call `session.save()` if `original_model` was non-empty and actually changed.
Adds a test asserting `save_calls == []` when `session.model is None`.
No behavior change for sessions with a real model (the primary use case of #751 is unaffected).
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