fix: clear last_pinned_prefix_hash on model/provider switch

clear_model_scoped_telemetry() now resets last_pinned_prefix_hash to
None so /cache stats does not show the previous model/provider cache
scope's fingerprint after a switch.

crates/tui/src/tui/app.rs

@@ -1631,6 +1631,7 @@ impl App {
         self.session.last_prompt_cache_miss_tokens = None;
         self.session.last_reasoning_replay_tokens = None;
         self.session.turn_cache_history.clear();
+        self.last_pinned_prefix_hash = None;
     }
 
     pub fn tr(&self, id: MessageId) -> &'static str {