[cli] Address review feedback on --prebuilt-dir

- upload_using_ltchiptool: rename unused config arg to _config and note
  the signature parity with upload_using_platformio/upload_using_picotool
  so dispatch stays symmetric.
- upload_program: explicit exit_code = 1 when _rp2040_serial_reset_to_bootsel
  fails, instead of relying on the function-level default.
- upload_using_picotool: distinct error message for the prebuilt-dir case
  that names both candidates (idedata ELF + prebuilt firmware) instead of
  just pointing at the missing ELF.
- _load_idedata: document the prebuilt-dir idedata.json contract more
  loudly (absolute paths under prebuilt_dir, no schema validation, dashboard
  owns the rewrites).
- --prebuilt-dir help text: drop the placeholder docs URL; describe the
  dashboard-internal intent inline so users who hit the flag in --help
  understand they don't want it.
- New test: upload_using_esptool with ESP-IDF toolchain + --prebuilt-dir
  uses <prebuilt-dir>/firmware.factory.bin at offset 0x0.

Issue: esphome/device-builder#572

tests/unit_tests/test_main.py

@@ -1343,6 +1343,41 @@ def test_upload_using_esptool_with_file_path(
     assert firmware_path.endswith("custom_firmware.bin")
 
 
+def test_upload_using_esptool_idf_prebuilt_factory_bin(
+    tmp_path: Path,
+    mock_run_external_command_main: Mock,
+) -> None:
+    """ESP-IDF builds flash a single ``firmware.factory.bin`` (bootloader +
+    partitions + app bundled) at offset 0x0. With --prebuilt-dir set the
+    esptool helper must pick up ``<prebuilt-dir>/firmware.factory.bin``
+    instead of ``CORE.relative_build_path("build", "firmware.factory.bin")``
+    so the dashboard can flash an ESP-IDF device without a local build."""
+    setup_core(platform=PLATFORM_ESP32, tmp_path=tmp_path, name="test")
+    CORE.toolchain = Toolchain.ESP_IDF
+    CORE.data[KEY_ESP32] = {KEY_VARIANT: VARIANT_ESP32}
+
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    factory_bin = prebuilt / "firmware.factory.bin"
+    factory_bin.write_bytes(b"\x00" * 1024)
+    CORE.prebuilt_dir = prebuilt
+
+    config = {CONF_ESPHOME: {"platformio_options": {}}}
+
+    result = upload_using_esptool(config, "/dev/ttyUSB0", None, None)
+
+    assert result == 0
+    cmd_list = list(mock_run_external_command_main.call_args[0][1:])
+
+    # ESP-IDF prebuilt: single image at offset 0x0 pointing at the prebuilt
+    # factory file, not the local build tree's <build>/firmware.factory.bin.
+    write_flash_idx = cmd_list.index("write-flash")
+    offset_idx = write_flash_idx + 4
+    assert cmd_list[offset_idx] == "0x0"
+    path_arg = cmd_list[offset_idx + 1]
+    assert path_arg == str(factory_bin)
+
+
 @pytest.mark.parametrize(
     "platform,device",
     [