fix: validate LED group_repeats 1-254 and tolerate raw 0xFF on parse (M11)

src/opendisplay/models/led_flash.py

@@ -47,8 +47,10 @@ def __post_init__(self) -> None:
         _check_nibble("mode", self.mode)
         if not 1 <= self.brightness <= 16:
             raise ValueError(f"brightness out of range: {self.brightness} (must be 1-16)")
-        if self.group_repeats is not None and not 1 <= self.group_repeats <= 255:
-            raise ValueError(f"group_repeats out of range: {self.group_repeats} (must be 1-255 or None for infinite)")
+        # Encoded as group_repeats-1; raw 0xFE is the firmware's infinite
+        # sentinel, so 255 finite repeats would encode to 0xFE and loop forever.
+        if self.group_repeats is not None and not 1 <= self.group_repeats <= 254:
+            raise ValueError(f"group_repeats out of range: {self.group_repeats} (must be 1-254 or None for infinite)")
         _check_u8("reserved", self.reserved)
 
     @classmethod
@@ -125,7 +127,10 @@ def from_bytes(cls, data: bytes) -> LedFlashConfig:
         mode = data[0] & 0x0F
         brightness = ((data[0] >> 4) & 0x0F) + 1
         group_raw = data[10]
-        group_repeats = None if group_raw == 0xFE else (group_raw + 1)
+        # 0xFE is the firmware's infinite sentinel; 0xFF (which our encoder never
+        # emits) is a firmware-valid unbounded value — treat both as infinite so
+        # parsing a device's payload never raises. Finite values decode as raw+1.
+        group_repeats = None if group_raw in (0xFE, 0xFF) else (group_raw + 1)
 
         return cls(
             mode=mode,

tests/unit/test_models_led_flash.py

@@ -72,3 +72,20 @@ def test_led_flash_config_rejects_invalid_values() -> None:
 
     with pytest.raises(ValueError, match="color out of range"):
         LedFlashStep(color=256)
+
+
+def test_group_repeats_255_rejected_to_avoid_infinite_sentinel() -> None:
+    # 255 would encode to raw 0xFE (the infinite sentinel) and loop forever (M11).
+    with pytest.raises(ValueError, match="group_repeats out of range"):
+        LedFlashConfig(group_repeats=255)
+
+
+def test_group_repeats_254_is_max_finite() -> None:
+    cfg = LedFlashConfig(group_repeats=254)
+    assert cfg.to_bytes()[10] == 253  # raw = group_repeats - 1
+
+
+def test_from_bytes_accepts_raw_0xff_without_raising() -> None:
+    payload = bytes([0x70, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF, 0])
+    cfg = LedFlashConfig.from_bytes(payload)  # must not raise
+    assert cfg.group_repeats is None