Close coverage gap and fix phantom lamp switch for TP2X_DA-KS-WALLOVEN (issue #300)

/diagnosis/vs/0 was the dump's only unbound href, now covered via
dishwasher.DIAGNOSIS (same shape already reused by airconditioner.py).

This steam-oven-class board's /mode/vs/0 options[] carries no UpperLamp_
token at all, unlike the NV7000BS-class board LAMP was proven against --
LAMP had no exists_fn, so it registered anyway, always read Off, and any
write to it was a no-op the device had no reason to honor. Gives it the
same options-token exists_fn gate issue #183 already added to
fast_preheat/natural_steam/energy_saving/cooktop_on_alert.

custom_components/localthings/registry/by_type/oven.py

@@ -1,6 +1,6 @@
 """Oven device registry."""
 
-from ..capabilities import common, ignored, oven
+from ..capabilities import common, dishwasher, ignored, oven
 from ._base import DeviceRegistry, _build
 
 REGISTRY = DeviceRegistry(
@@ -18,6 +18,9 @@ REGISTRY = DeviceRegistry(
             oven.OVEN_CONNECTED,
             oven.OVEN_SPEC,
             oven.OVEN_RECIPE_COOK,
+            # issue #300: /diagnosis/vs/0 is the same diagnosisStart shape
+            # dishwasher.py and airconditioner.py already reuse.
+            dishwasher.DIAGNOSIS,
         ]
     ),
 )

custom_components/localthings/registry/capabilities/oven.py

@@ -401,10 +401,17 @@ OVEN_MODE = Capability(
             value_fn=lambda v: v[0] if v else None,
             write_fn=_oven_mode_write,
         ),
+        # No exists_fn on the NV7000BS-class board this was proven against
+        # (UpperLamp_ is always in its options[]) -- but issue #300's
+        # steam-oven-class WALLOVEN board's options[] has no UpperLamp_
+        # token at all, so this was a phantom, always-off, write-does-
+        # nothing switch there. Same fastpreheat/NaturalSteam-class gap
+        # issue #183 already fixed on the other switches below.
         SwitchDesc(
             key="lamp",
             field="x.com.samsung.da.options",
             icon="mdi:track-light",
+            exists_fn=_has_option("UpperLamp"),
             value_fn=lambda opts: _option_value(opts, "UpperLamp") == "On",
             write_fn=_option_switch_write("UpperLamp"),
         ),