feat(mctp-rs): add MctpMessageBuffer::body() + message_type() accessors

Expose the message body slice and message_type byte on MctpMessageBuffer
so callers can read raw bytes without going through MctpMessage::parse_as.

Use case: a consumer wants to parse a small fixed-layout payload (e.g.,
4 little-endian u32 fields for ACPI BST) without defining a full
MctpMessageTrait impl just to access self.message_buffer.rest.

Pure additive: no public API removed; existing parse_as / can_parse_as
paths unchanged.

Author: dymk

mctp-rs/src/lib.rs

@@ -211,6 +211,22 @@ pub struct MctpMessageBuffer<'buffer> {
     rest: &'buffer [u8],
 }
 
+impl<'buffer> MctpMessageBuffer<'buffer> {
+    /// The raw message body bytes (after the message-type byte).
+    ///
+    /// Use when the caller wants to parse the body without going through
+    /// `MctpMessage::parse_as` (e.g., when the body is a small fixed-layout
+    /// payload and a typed `MctpMessageTrait` impl would be overkill).
+    pub fn body(&self) -> &'buffer [u8] {
+        self.rest
+    }
+
+    /// The MCTP message-type byte (e.g., `0x7E` for VendorDefinedPci).
+    pub fn message_type(&self) -> u8 {
+        self.message_type
+    }
+}
+
 impl<'buffer, M: MctpMedium> MctpMessage<'buffer, M> {
     pub fn can_parse_as<P: MctpMessageTrait<'buffer>>(&self) -> bool {
         self.message_buffer.message_type == P::MESSAGE_TYPE