Fix SpiAdapter.Read buffer length mismatch in Mcp23xxx TransferFullDuplex

src/devices/Mcp23xxx/SpiAdapter.cs

@@ -37,18 +37,15 @@ public SpiAdapter(SpiDevice device, int deviceAddress)
             /// <inheritdoc/>
             public override void Read(byte registerAddress, Span<byte> buffer)
             {
-                // Include OpCode and Register Address.
-                Span<byte> writeBuffer = stackalloc byte[]
-                {
-                    GetOpCode(_deviceAddress, isReadCommand: true),
-                    registerAddress
-                };
+                int fullLength = buffer.Length + 2;
 
-                Span<byte> readBuffer = stackalloc byte[buffer.Length + 2];
+                // Include OpCode and Register Address.
+                // writeBuffer must be the same length as readBuffer for TransferFullDuplex.
+                Span<byte> writeBuffer = stackalloc byte[fullLength];
+                writeBuffer[0] = GetOpCode(_deviceAddress, isReadCommand: true);
+                writeBuffer[1] = registerAddress;
 
-                // Should this also contain the op code and register?
-                // Why are we transferring full duplex if we only really
-                // need to read?
+                Span<byte> readBuffer = stackalloc byte[fullLength];
                 _device.TransferFullDuplex(writeBuffer, readBuffer);
 
                 // First 2 bytes are from sending OpCode and Register Address.

src/devices/Mcp23xxx/tests/Mcp23xxxTest.cs

@@ -71,7 +71,10 @@ public SpiDeviceMock(int ports)
 
             public override void TransferFullDuplex(ReadOnlySpan<byte> writeBuffer, Span<byte> readBuffer)
             {
-                Write(writeBuffer);
+                // Only pass the command bytes (OpCode + RegisterAddress) to Write.
+                // Remaining bytes in writeBuffer are don't-care padding for full-duplex reads
+                // and should not be written as register data.
+                Write(writeBuffer.Slice(0, Math.Min(writeBuffer.Length, 2)));
                 Read(readBuffer);
             }