[message] update SetOffset() and MoveOffset() to clamp offset (#12522)

Updates `SetOffset()` to ensure the offset is always clamped to the
current message length. This guarantees that the offset remains valid
and does not exceed the message size.

Updates `MoveOffset()` to accept `int16_t` as the delta and ensures
the calculated new offset is clamped within `0` and
`NumericLimits<uint16_t>::kMax` before setting it.

Updates `SetLength()` to rely on the new `SetOffset()` behavior to
automatically adjust the offset when the message length is reduced.

src/core/common/message.cpp

@@ -281,10 +281,7 @@ Error Message::SetLength(uint16_t aLength)
     GetMetadata().mLength = aLength;
 
     // Correct the offset in case shorter length is set.
-    if (GetOffset() > aLength)
-    {
-        SetOffset(aLength);
-    }
+    SetOffset(GetOffset());
 
 exit:
     return error;
@@ -314,18 +311,16 @@ uint8_t Message::GetBufferCount(void) const
     return rval;
 }
 
-void Message::MoveOffset(int aDelta)
+void Message::MoveOffset(int16_t aDelta)
 {
-    OT_ASSERT(GetOffset() + aDelta <= GetLength());
-    GetMetadata().mOffset += static_cast<int16_t>(aDelta);
-    OT_ASSERT(GetMetadata().mOffset <= GetLength());
+    int32_t newOffset = static_cast<int32_t>(GetOffset()) + aDelta;
+
+    newOffset = Clamp<int32_t>(newOffset, 0, NumericLimits<uint16_t>::kMax);
+
+    SetOffset(static_cast<uint16_t>(newOffset));
 }
 
-void Message::SetOffset(uint16_t aOffset)
-{
-    OT_ASSERT(aOffset <= GetLength());
-    GetMetadata().mOffset = aOffset;
-}
+void Message::SetOffset(uint16_t aOffset) { GetMetadata().mOffset = Min(aOffset, GetLength()); }
 
 uint16_t Message::DetermineLengthAfterOffset(void) const
 {

src/core/common/message.hpp

@@ -528,7 +528,7 @@ public:
      *
      * @param[in]  aDelta  The number of bytes to move the current offset, which may be positive or negative.
      */
-    void MoveOffset(int aDelta);
+    void MoveOffset(int16_t aDelta);
 
     /**
      * Sets the byte offset within the message.

src/core/instance/instance.hpp

@@ -54,6 +54,8 @@
 #include "common/log.hpp"
 #include "common/message.hpp"
 #include "common/non_copyable.hpp"
+#include "common/num_utils.hpp"
+#include "common/numeric_limits.hpp"
 #include "common/random.hpp"
 #include "common/serial_number.hpp"
 #include "common/string.hpp"