Add request retries and nominal capacity override number entity

Retries (bluetooth_handler):
- request() now retries up to 3× with 0.5s pause between attempts
- Tries Write With Response first, falls back to Write Without Response
  automatically if the characteristic rejects it — handles both BMS variants

Number entity (number.py):
- "Nominal Capacity (Override)" lets user correct a stale BMS capacity value
  (e.g. after a cell upgrade) without PC software
- Value is restored across HA restarts via RestoreEntity
- Immediately patches coordinator.data so the sensor reflects it without
  waiting for the next poll

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

custom_components/xiaoxiang_bms/bluetooth_handler.py

@@ -123,22 +123,52 @@ class BmsBluetoothHandler:
     # Request / response
     # ------------------------------------------------------------------
 
-    async def request(self, command: bytes, timeout: float = 5.0) -> bytes | None:
-        """Send a command frame and wait for the corresponding response frame."""
+    async def request(
+        self,
+        command: bytes,
+        timeout: float = 5.0,
+        retries: int = 3,
+    ) -> bytes | None:
+        """Send a command frame and wait for the corresponding response frame.
+
+        Retries up to `retries` times with a short pause between attempts to
+        handle occasional BLE packet loss or a slow BMS response.
+        Tries Write With Response first; if that raises a GATT error the BMS
+        likely only supports Write Without Response, so we fall back silently.
+        """
         async with self._lock:
-            self._response_event.clear()
-            self._response_data = None
-            try:
-                await self._client.write_gatt_char(TX_CHAR_UUID, command, response=True)
-            except BleakError as exc:
-                _LOGGER.error("BLE write failed: %s", exc)
-                return None
-            try:
-                await asyncio.wait_for(self._response_event.wait(), timeout)
-                return self._response_data
-            except asyncio.TimeoutError:
-                _LOGGER.warning("BMS response timeout (cmd=%s)", command.hex())
-                return None
+            for attempt in range(1, retries + 1):
+                self._response_event.clear()
+                self._response_data = None
+                try:
+                    await self._client.write_gatt_char(
+                        TX_CHAR_UUID, command, response=True
+                    )
+                except BleakError:
+                    # Characteristic may not support Write With Response — try without
+                    try:
+                        await self._client.write_gatt_char(
+                            TX_CHAR_UUID, command, response=False
+                        )
+                    except BleakError as exc:
+                        _LOGGER.error("BLE write failed (attempt %d/%d): %s",
+                                      attempt, retries, exc)
+                        if attempt < retries:
+                            await asyncio.sleep(0.5)
+                        continue
+
+                try:
+                    await asyncio.wait_for(self._response_event.wait(), timeout)
+                    return self._response_data
+                except asyncio.TimeoutError:
+                    _LOGGER.warning(
+                        "BMS response timeout (cmd=0x%s, attempt %d/%d)",
+                        command.hex(), attempt, retries,
+                    )
+                    if attempt < retries:
+                        await asyncio.sleep(0.5)
+
+        return None
 
     # ------------------------------------------------------------------
     # Frame parsers