The route dict in _persist_model_switch_to_session used filtering
(omits falsy values) while the top-level keys used (writes
explicit None to trigger deletion in _merge_model_config_json). This
asymmetry meant stale keys from a previous /model switch survived in
the nested gateway_runtime dict even after the fix in #85261 that
properly deleted them from the top-level keys.
Fix: build the route dict with and derive the top-level keys
from **route so both shapes always use identical deletion semantics.
Also filter None values in session_gateway_runtime's reader since
gateway_runtime is replaced as a whole dict (not deep-merged), so None
values written by the persist path survive in the nested dict.
Found by /simplify-code 3-reviewer review on #85261 (all 3 reviewers
converged on the route dict asymmetry as the verdict-relevant finding).
9cb456a9b9