[nrf52] Honor --prebuilt-dir for mcumgr/BLE OTA uploads

nRF52 has its own upload_program that runs before the default dispatch
in esphome.__main__. The mcumgr/BLE OTA path reads the MCUboot-signed
update image from CORE.relative_pioenvs_path(name, 'zephyr',
'app_update.bin'), which assumes a local Zephyr build tree.

Consult CORE.prebuilt_artifact_path('app_update.bin') first so the
dashboard's transparent BLE install on a Bluetooth proxy can flash a
prebuilt update image without compiling locally.

Serial uploads on non-MCUboot bootloaders still go through
adafruit-nrfutil via _upload_using_platformio and are out of scope here;
they need their own bypass to work with --prebuilt-dir (tracked
separately).

Issue: esphome/device-builder#572

esphome/components/nrf52/__init__.py

@@ -436,9 +436,17 @@ def upload_program(config: ConfigType, args, host: str) -> bool:
         mcumgr_device = host
 
     if mcumgr_device:
-        firmware = Path(
-            CORE.relative_pioenvs_path(CORE.name, "zephyr", "app_update.bin")
-        ).resolve()
+        # `esphome upload --prebuilt-dir <path>` ships the MCUboot-signed
+        # update image at the root of the prebuilt directory; prefer that so
+        # the dashboard's transparent BLE install on a Bluetooth proxy works
+        # without a local Zephyr build tree.
+        prebuilt = CORE.prebuilt_artifact_path("app_update.bin")
+        if prebuilt is not None:
+            firmware = prebuilt.resolve()
+        else:
+            firmware = Path(
+                CORE.relative_pioenvs_path(CORE.name, "zephyr", "app_update.bin")
+            ).resolve()
         asyncio.run(smpmgr_upload(mcumgr_device, firmware))
         return True  # Handled: mcumgr OTA upload
 

tests/unit_tests/components/test_nrf52.py

@@ -0,0 +1,52 @@
+"""Tests for the nrf52 component's custom upload_program."""
+
+from __future__ import annotations
+
+from pathlib import Path
+from unittest.mock import AsyncMock, MagicMock, patch
+
+from esphome.core import CORE
+
+
+def test_nrf52_upload_program_prebuilt_dir_for_ble_ota(tmp_path: Path) -> None:
+    """`esphome upload --prebuilt-dir <path>` for an nRF52 Bluetooth proxy
+    ships the MCUboot-signed update at <prebuilt-dir>/app_update.bin so the
+    dashboard's transparent BLE install doesn't need a local Zephyr build
+    tree. Verify the nrf52 upload_program picks up that path instead of
+    `CORE.relative_pioenvs_path(..., "zephyr", "app_update.bin")`.
+    """
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    expected_firmware = prebuilt / "app_update.bin"
+    expected_firmware.write_bytes(b"signed-mcuboot-image")
+    CORE.prebuilt_dir = prebuilt
+    CORE.name = "bt-proxy"
+
+    captured_firmware: list[Path] = []
+
+    async def fake_upload(device: str, firmware: Path) -> None:
+        captured_firmware.append(firmware)
+
+    # Patch the heavy nrf52 surface so we exercise only the firmware-path
+    # resolution: BLE scan returns a fake device, smpmgr_upload records the
+    # firmware path it would have flashed.
+    with (
+        patch(
+            "esphome.components.nrf52.ota.smpmgr_scan",
+            new=AsyncMock(return_value="AA:BB:CC:DD:EE:FF"),
+        ),
+        patch(
+            "esphome.components.nrf52.ota.smpmgr_upload",
+            new=fake_upload,
+        ),
+        patch(
+            "esphome.components.nrf52.ble_logger.is_mac_address",
+            return_value=False,
+        ),
+    ):
+        from esphome.components.nrf52 import upload_program
+
+        handled = upload_program({}, MagicMock(), "BLE")
+
+    assert handled is True
+    assert captured_firmware == [expected_firmware.resolve()]