[api] Avoid heap string copies in Home Assistant state subscription callbacks (#12506)

esphome/components/api/api_connection.cpp

@@ -1692,10 +1692,18 @@ void APIConnection::on_home_assistant_state_response(const HomeAssistantStateRes
       continue;
     }
 
-    // Create temporary string for callback (callback takes const std::string &)
-    // Handle empty state
-    std::string state(!msg.state.empty() ? msg.state.c_str() : "", msg.state.size());
-    it.callback(state);
+    // Create null-terminated state for callback (parse_number needs null-termination)
+    // HA state max length is 255, so 256 byte buffer covers all cases
+    char state_buf[256];
+    size_t copy_len = msg.state.size();
+    if (copy_len >= sizeof(state_buf)) {
+      copy_len = sizeof(state_buf) - 1;  // Truncate to leave space for null terminator
+    }
+    if (copy_len > 0) {
+      memcpy(state_buf, msg.state.c_str(), copy_len);
+    }
+    state_buf[copy_len] = '\0';
+    it.callback(StringRef(state_buf, copy_len));
   }
 }
 #endif