fix(gateway): /usage billing route follows the most recent model too

`_persisted_billing_route` (idle `/usage` account-limits lookup) was the last
reader of the lifetime-dominant route, so it queried the retired provider's
account after a switch. Point it at `get_recent_session_model_route` and
delete the dominant query, which no longer has a caller.

hermes_state_sessions.py

@@ -744,29 +744,11 @@ class SessionSessionsMixin:
         )
         return self._session_row_dict(row) if row else None
 
-    def get_dominant_session_model_route(self, session_id: str) -> Optional[Dict[str, Any]]:
-        """Main-loop model route that served most API calls (``session_model_usage`` keeps the coherent
-        per-call tuple; ``sessions`` mixes route changes)."""
-        self.flush_token_counts()
-        row = self._read_one(
-            """SELECT model, billing_provider, billing_base_url, billing_mode,
-                      api_call_count
-                 FROM session_model_usage
-                WHERE session_id = ?
-                  AND task = ''
-                  AND model <> 'unknown'
-                  AND billing_provider <> ''
-                ORDER BY api_call_count DESC,
-                         (input_tokens + output_tokens + cache_read_tokens +
-                          cache_write_tokens + reasoning_tokens) DESC,
-                         last_seen DESC
-                LIMIT 1""",
-            (session_id,),
-        )
-        return dict(row) if row else None
-
     def get_recent_session_model_route(self, session_id: str) -> Optional[Dict[str, Any]]:
-        """Most recently used main-loop model route as one coherent per-call tuple."""
+        """Most recently used main-loop model route as one coherent per-call tuple
+        (``session_model_usage`` keeps model+provider together; ``sessions`` mixes route changes).
+        Recency, not lifetime call count: on a long session a route retired weeks ago can hold the
+        highest ``api_call_count`` forever, and /status and /usage would keep calling it current."""
         self.flush_token_counts()
         row = self._read_one(
             """SELECT model, billing_provider, billing_base_url, billing_mode,