Fix BlueZ GATT server registration and OBD-II mode 03/04 parsing

Add @dbus_property(PropertyAccess.READ) to GattServiceIface,
GattCharacteristicIface, and GattDescriptorIface so BlueZ can
validate objects via the standard Properties interface — fixes
"No valid service object found" error on RegisterApplication.

Also fix ELM327 emulator rejecting 2-char OBD commands (mode 03
for DTCs and mode 04 for clear) by padding to 4 chars.

src/mcbluetooth/gatt_server.py

@@ -24,7 +24,7 @@ from typing import Any
 
 from dbus_fast import BusType, Message, MessageType, Variant
 from dbus_fast.aio import MessageBus
-from dbus_fast.service import ServiceInterface, dbus_property, method
+from dbus_fast.service import PropertyAccess, ServiceInterface, dbus_property, method
 
 log = logging.getLogger(__name__)
 
@@ -130,11 +130,22 @@ class GattApplication(ServiceInterface):
 class GattServiceIface(ServiceInterface):
     """org.bluez.GattService1 — exported at each service path.
 
-    No D-Bus methods; properties served via GetManagedObjects only.
+    Properties must be exposed via @dbus_property so BlueZ can validate
+    the service object through the standard Properties interface.
     """
 
-    def __init__(self):
+    def __init__(self, uuid: str, primary: bool):
         super().__init__("org.bluez.GattService1")
+        self._uuid = uuid
+        self._primary = primary
+
+    @dbus_property(PropertyAccess.READ)
+    def UUID(self) -> "s":  # noqa: F821
+        return self._uuid
+
+    @dbus_property(PropertyAccess.READ)
+    def Primary(self) -> "b":  # noqa: F821
+        return self._primary
 
 
 class GattCharacteristicIface(ServiceInterface):
@@ -142,6 +153,7 @@ class GattCharacteristicIface(ServiceInterface):
 
     ReadValue returns the stored value. WriteValue stores the value AND
     records a WriteEvent for LLM polling (like agent.py's pending_requests).
+    Properties exposed via @dbus_property for BlueZ introspection.
     """
 
     def __init__(self, char: ServerCharacteristic, manager: "GattServerManager"):
@@ -149,6 +161,18 @@ class GattCharacteristicIface(ServiceInterface):
         self._char = char
         self._manager = manager
 
+    @dbus_property(PropertyAccess.READ)
+    def UUID(self) -> "s":  # noqa: F821
+        return self._char.uuid
+
+    @dbus_property(PropertyAccess.READ)
+    def Service(self) -> "o":  # noqa: F821
+        return f"{APP_BASE_PATH}/{self._char.service_id}"
+
+    @dbus_property(PropertyAccess.READ)
+    def Flags(self) -> "as":  # noqa: F821
+        return self._char.flags
+
     @method()
     def ReadValue(self, options: "a{sv}") -> "ay":  # noqa: F821
         offset = 0
@@ -189,12 +213,27 @@ class GattCharacteristicIface(ServiceInterface):
 
 
 class GattDescriptorIface(ServiceInterface):
-    """org.bluez.GattDescriptor1 — handles reads/writes on descriptors."""
+    """org.bluez.GattDescriptor1 — handles reads/writes on descriptors.
+
+    Properties exposed via @dbus_property for BlueZ introspection.
+    """
 
     def __init__(self, desc: ServerDescriptor):
         super().__init__("org.bluez.GattDescriptor1")
         self._desc = desc
 
+    @dbus_property(PropertyAccess.READ)
+    def UUID(self) -> "s":  # noqa: F821
+        return self._desc.uuid
+
+    @dbus_property(PropertyAccess.READ)
+    def Characteristic(self) -> "o":  # noqa: F821
+        return f"{APP_BASE_PATH}/{self._desc.char_id}"
+
+    @dbus_property(PropertyAccess.READ)
+    def Flags(self) -> "as":  # noqa: F821
+        return self._desc.flags
+
     @method()
     def ReadValue(self, options: "a{sv}") -> "ay":  # noqa: F821
         offset = 0
@@ -226,15 +265,15 @@ class LEAdvertisementIface(ServiceInterface):
     def Release(self) -> None:
         log.info("GATT server: advertisement released by BlueZ")
 
-    @dbus_property()
+    @dbus_property(PropertyAccess.READ)
     def Type(self) -> "s":  # noqa: F821
         return self._type
 
-    @dbus_property()
+    @dbus_property(PropertyAccess.READ)
     def LocalName(self) -> "s":  # noqa: F821
         return self._local_name
 
-    @dbus_property()
+    @dbus_property(PropertyAccess.READ)
     def ServiceUUIDs(self) -> "as":  # noqa: F821
         return self._service_uuids
 
@@ -277,7 +316,7 @@ class GattServerManager:
             uuid=uuid,
             primary=primary,
         )
-        svc.dbus_obj = GattServiceIface()
+        svc.dbus_obj = GattServiceIface(uuid, primary)
         self._services[service_id] = svc
 
         log.info("GATT server: added service %s (UUID=%s)", service_id, uuid)

src/mcbluetooth/tools/bt_elm327_emu.py

@@ -231,7 +231,10 @@ class ELM327Emulator:
         cr = "\r\n" if self.linefeed else "\r"
         cmd = cmd.replace(" ", "")
 
-        if len(cmd) < 4:
+        # Modes 03/04 don't require a PID — pad to 4 chars
+        if len(cmd) == 2:
+            cmd += "00"
+        elif len(cmd) < 4:
             return f"?{cr}{cr}>"
 
         try: