fix: increment request_count in least_used pool strategy

The least_used strategy selected entries via min(request_count) but
never incremented the counter. All entries stayed at count=0, so the
strategy degenerated to fill_first behavior with no actual load balancing.

Now increments request_count after each selection and persists the update.

agent/credential_pool.py

@@ -739,8 +739,11 @@ class CredentialPool:
 
         if self._strategy == STRATEGY_LEAST_USED and len(available) > 1:
             entry = min(available, key=lambda e: e.request_count)
+            # Increment usage counter so subsequent selections distribute load
+            updated = replace(entry, request_count=entry.request_count + 1)
+            self._replace_entry(entry, updated)
             self._current_id = entry.id
-            return entry
+            return updated
 
         if self._strategy == STRATEGY_ROUND_ROBIN and len(available) > 1:
             entry = available[0]