[usb_host] Fix EventPool/LockFreeQueue sizing off-by-one (#14896)

esphome/components/usb_host/usb_host.h

@@ -144,7 +144,10 @@ class USBClient : public Component {
   // Lock-free event queue and pool for USB task to main loop communication
   // Must be public for access from static callbacks
   LockFreeQueue<UsbEvent, USB_EVENT_QUEUE_SIZE> event_queue;
-  EventPool<UsbEvent, USB_EVENT_QUEUE_SIZE> event_pool;
+  // Pool sized to queue capacity (SIZE-1) because LockFreeQueue<T,N> is a ring
+  // buffer that holds N-1 elements. This guarantees allocate() returns nullptr
+  // before push() can fail, preventing a pool slot leak.
+  EventPool<UsbEvent, USB_EVENT_QUEUE_SIZE - 1> event_pool;
 
  protected:
   // Process USB events from the queue. Returns true if any work was done.

esphome/components/usb_host/usb_host_client.cpp

@@ -193,7 +193,8 @@ static void client_event_cb(const usb_host_client_event_msg_t *event_msg, void *
       return;
   }
 
-  // Push to lock-free queue (always succeeds since pool size == queue size)
+  // Push always succeeds: pool is sized to queue capacity (SIZE-1), so if
+  // allocate() returned non-null, the queue cannot be full.
   client->event_queue.push(event);
 
   // Re-enable component loop to process the queued event