fix(falcon): name prefix in __init__ so the device server stops dropping it

The __init__ added in the previous commit took (*args, **kwargs). The device
server builds a device's init kwargs by intersecting the deviceConfig keys
with the NAMED parameters of the class signature
(bec_server/device_server/devices/devicemanager.py:469-475), so 'prefix' was
silently discarded and the Falcon was constructed with an empty prefix.

Every signal then pointed at a bare suffix -- HDF1:FilePath_RBV instead of
X12SA-SITORO:HDF1:FilePath_RBV -- and instantiation failed with
"TimeoutError: Failed to connect to all signals" listing several hundred PVs.
That reads like an unreachable IOC, which is how it was diagnosed at the
beamline for two hours, while caget from the same host worked perfectly.

The signature now names name, prefix, scan_info, device_manager and
xml_file_name explicitly, matching DDG1. Two tests guard it: one asserts every
deviceConfig key is a named parameter, the other that a configured prefix
reaches the signal PV names.

Reported-by: Klaus Wakonig <klaus.wakonig@psi.ch>

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KLnmUurqcNd1FiDY5M2uZr

csaxs_bec/devices/epics/falcon_csaxs.py

@@ -4,6 +4,7 @@ import enum
 import os
 import threading
 import time
+from typing import TYPE_CHECKING
 from typing import Literal
 
 from bec_lib.file_utils import get_full_path
@@ -17,6 +18,9 @@ from ophyd_devices.interfaces.base_classes.psi_device_base import PSIDeviceBase
 
 from csaxs_bec.devices.utils.utils import fetch_scan_info
 
+if TYPE_CHECKING:  # pragma: no cover
+    from bec_lib.devicemanager import DeviceManagerBase, ScanInfo
+
 logger = bec_logger.logger
 
 # NOTE Layout of the written HDF5 file, resolved by the IOC relative to its own working
@@ -82,16 +86,35 @@ class FalconcSAXS(PSIDeviceBase, FalconControl):
 
     file_event = Cpt(FileEventSignal, name="file_event")
 
-    def __init__(self, *args, xml_file_name: str | None = None, **kwargs):
+    def __init__(
+        self,
+        name: str,
+        prefix: str = "",
+        scan_info: "ScanInfo | None" = None,
+        device_manager: "DeviceManagerBase | None" = None,
+        xml_file_name: str | None = None,
+        **kwargs,
+    ):
         """
+        Every deviceConfig key must appear here by name. The device server builds the
+        init kwargs by intersecting the config keys with the class signature's named
+        parameters, so a bare *args/**kwargs signature silently drops 'prefix' and the
+        device is built with empty PV names.
+
         Args:
+            name: device name.
+            prefix: EPICS prefix, e.g. 'X12SA-SITORO:'.
+            scan_info: scan info object.
+            device_manager: BEC device manager.
             xml_file_name: HDF5 layout description, as the IOC resolves it. Defaults to
                 DEFAULT_XML_FILE_NAME; pass "" to use the plugin's built-in layout.
         """
         self._xml_file_name_config = (
             DEFAULT_XML_FILE_NAME if xml_file_name is None else xml_file_name
         )
-        super().__init__(*args, **kwargs)
+        super().__init__(
+            name=name, prefix=prefix, scan_info=scan_info, device_manager=device_manager, **kwargs
+        )
 
     def on_init(self) -> None:
         """Initialize Falcon Sitoro detector"""

tests/tests_devices/test_falcon_csaxs.py

@@ -1,4 +1,5 @@
 # pylint: skip-file
+import inspect
 import os
 import threading
 from typing import Generator
@@ -360,3 +361,32 @@ def test_falcon_accepts_a_valid_layout_silently(mock_det: FalconcSAXS):
     ):
         assert falcon._check_xml_layout(timeout=0.05) is True
         mock_logger.error.assert_not_called()
+
+
+def test_falcon_init_names_every_device_config_key():
+    """The device server drops config keys that are not named parameters.
+
+    bec_server's devicemanager builds the init kwargs by intersecting the deviceConfig
+    keys with the class signature's named parameters. A *args/**kwargs signature
+    therefore silently discards 'prefix', and every PV is built without it -- which
+    presents as 'failed to connect to all signals' rather than as a config error.
+    """
+    params = inspect.signature(FalconcSAXS).parameters
+    for key in ("name", "prefix", "scan_info", "device_manager", "xml_file_name"):
+        assert key in params, f"{key} must be a named parameter or the device server drops it"
+
+
+def test_falcon_keeps_the_configured_prefix():
+    """A prefix passed as the device server passes it must reach the signals."""
+    with patched_device(
+        FalconcSAXS,
+        name="falcon",
+        prefix="X12SA-SITORO:",
+        device_manager=DMMock(),
+        _mock_pv_initial_value=1,
+    ) as dev:
+        try:
+            assert dev.prefix == "X12SA-SITORO:"
+            assert dev.hdf5.file_path.pvname.startswith("X12SA-SITORO:")
+        finally:
+            dev.destroy()