When the user explicitly selects @provider:model from the picker,
_resolve_compatible_session_model() was stripping the prefix because
the hint matched the active provider (hint_matches_active=True → return bare_model, True).
This caused:
- The picker to snap back to the first duplicate entry on next render
- resolve_model_provider() to use the default provider instead of the
explicitly selected one, running the agent on the wrong backend
The hint_matches_active branch was intended for normalizing stale cross-
provider session models. But an @provider:model where the hint IS the
active provider is not stale — it is the user's deliberate selection.
Fix: return (model, False) so the full @provider:model survives to
resolve_model_provider() in config.py, which already handles it correctly.
Updates test_active_at_provider_session_model_preserved_with_hint and
adds test_issue1253_duplicate_model_id_active_provider_hint_preserved.
Closes#1253
bd8fc6a2e2