[message] improve scoping of Buffer constants (#11885)

This commit moves two global constants, `kBufferSize` and
`kNumBuffers`, into their respective class scopes to improve
encapsulation and avoid potential name conflicts.

The `kBufferSize` constant is moved into the `Buffer` class as
`Buffer::kSize`. As `kBufferSize` is a generic and commonly used
name, this change prevents potential symbol collisions.

Similarly, the `kNumBuffers` constant is moved into the `MessagePool`
class, as it is exclusively used within that class.

All usages of these constants have been updated throughout the
codebase to reflect their new names.

tests/unit/test_message.cpp

@@ -39,7 +39,7 @@ namespace ot {
 
 void TestMessage(void)
 {
-    static constexpr uint16_t kMaxSize    = (kBufferSize * 3 + 24);
+    static constexpr uint16_t kMaxSize    = (Buffer::kSize * 3 + 24);
     static constexpr uint16_t kOffsetStep = 101;
     static constexpr uint16_t kLengthStep = 21;