Fix timestamp ticks decoding mask

Thread defines the Timestamp Ticks field as 15 bits. The parser was using a mask of `0x7FF` (11 bits), which truncated higher tick values.

This changes the mask to `0x7FFF` to correctly decode the full 15-bit value.

Related to home-assistant/core#134306

Author: MartB

python_otbr_api/tlv_parser.py

@@ -115,7 +115,7 @@ def __post_init__(self) -> None:
         unpacked: int = struct.unpack("!Q", self.data)[0]
         self.authoritative = bool(unpacked & 1)
         self.seconds = unpacked >> 16
-        self.ticks = (unpacked >> 1) & 0x7FF
+        self.ticks = (unpacked >> 1) & 0x7FFF
 
 
 def _encode_item(item: MeshcopTLVItem) -> bytes:

tests/test_tlv_parser.py

@@ -156,3 +156,33 @@ def test_parse_tlv_error(tlv, error, msg) -> None:
     """Test the TLV parser error handling."""
     with pytest.raises(error, match=msg):
         parse_tlv(tlv)
+
+
+def test_timestamp_parsing_full_integrity() -> None:
+    """
+    Test parsing of a timestamp with mixed values for seconds, ticks, and authoritative.
+    
+    We construct a value to ensure no bit overlap:
+    - Seconds: 400 (0x190)
+    - Ticks: 32767 (0x7FFF, Max value to catch the masking bug)
+    - Authoritative: True (1)
+    
+    Hex Construction:
+    - Seconds (48 bits): 00 00 00 00 01 90
+    - Ticks/Auth (16 bits):
+      (Ticks << 1) | Auth
+      (0x7FFF << 1) | 1  =>  0xFFFE | 1  =>  0xFFFF
+    
+    Combined Hex: 000000000190FFFF
+    """
+    timestamp_data = bytes.fromhex("000000000190FFFF")
+    timestamp = Timestamp(MeshcopTLVType.ACTIVETIMESTAMP, timestamp_data)
+
+    # 1. Check Seconds: Ensures the upper 48 bits are shifted correctly
+    assert timestamp.seconds == 400
+
+    # 2. Check Ticks: Ensures the mask is 0x7FFF (32767)
+    assert timestamp.ticks == 32767
+
+    # 3. Check Authoritative: Ensures the lowest bit is read correctly
+    assert timestamp.authoritative is True