[posix] fix SPI platform driver sanity check boundaries (#13236)

This commit corrects the SPI frame sanity checks in the POSIX platform
driver (`spi_interface.cpp`).

Previously, the sanity checks compared `mSpiSlaveDataLen` and
`slaveAcceptLen` against `kMaxFrameSize` (8192). However,
`mSpiSlaveDataLen` is the payload size, which excludes the 5-byte SPI
frame header. If the slave advertised a data length of exactly
`kMaxFrameSize` (8192), it would pass the sanity check, but the
subsequent `DoSpiTransfer` would request a transfer length of
`kMaxFrameSize + kSpiFrameHeaderSize + alignment` (e.g. 8213 bytes).
This would cause an out-of-bounds read on `mSpiTxFrameBuffer` which is
sized `kMaxFrameSize + kSpiAlignAllowanceMax` (8208 bytes).

This commit updates the sanity checks to use
`kMaxFrameSize - kSpiFrameHeaderSize` as the maximum allowed payload
length, ensuring that worst-case transfers always fit within the
tx buffer allocation.

src/posix/platform/spi_interface.cpp

@@ -412,8 +412,9 @@ otError SpiInterface::PushPullSpi(void)
     txFrame.SetHeaderAcceptLen(0);
     txFrame.SetHeaderDataLen(0);
 
-    // Sanity check.
-    if (mSpiSlaveDataLen > kMaxFrameSize)
+    // Sanity check. The header `data_len` carries the payload length only
+    // (it excludes header size), so the largest valid value is the MTU.
+    if (mSpiSlaveDataLen > kMaxFrameSize - kSpiFrameHeaderSize)
     {
         mSpiSlaveDataLen = 0;
     }
@@ -517,7 +518,8 @@ otError SpiInterface::PushPullSpi(void)
         slaveAcceptLen   = rxFrame.GetHeaderAcceptLen();
         mSpiSlaveDataLen = rxFrame.GetHeaderDataLen();
 
-        if (!rxFrame.IsValid() || (slaveAcceptLen > kMaxFrameSize) || (mSpiSlaveDataLen > kMaxFrameSize))
+        if (!rxFrame.IsValid() || (slaveAcceptLen > kMaxFrameSize - kSpiFrameHeaderSize) ||
+            (mSpiSlaveDataLen > kMaxFrameSize - kSpiFrameHeaderSize))
         {
             mInterfaceMetrics.mTransferredGarbageFrameCount++;
             mSpiTxRefusedCount++;