[coap] remove Clone() method from Coap::Message (#12616)

This commit removes the redundant `Clone()` methods from `Coap::Message`.
These methods previously served to retain the `HeaderOffset` across the
cloned message, which was internally tracked by modifying `MeshDest`.
Since `ot::Message::Clone()` natively preserves the `MeshDest` property,
the specialized `Coap::Message::Clone()` wrappers are unnecessary.
Callers now directly use `ot::Message::Clone()` and type-cast the result
using `AsCoapMessagePtr()`.

src/core/coap/coap.cpp

@@ -1024,11 +1024,10 @@ Error CoapBase::CacheLastBlockResponse(Message *aResponse)
 
     FreeLastBlockResponse();
 
-    if ((mLastResponse = aResponse->Clone()) == nullptr)
-    {
-        error = kErrorNoBufs;
-    }
+    mLastResponse = AsCoapMessagePtr(aResponse->Clone());
+    VerifyOrExit(mLastResponse != nullptr, error = kErrorNoBufs);
 
+exit:
     return error;
 }
 
@@ -1529,7 +1528,7 @@ Error CoapBase::PendingRequests::AddClone(const Message &aMessage, uint16_t aCop
 {
     Error error = kErrorNone;
 
-    aRequest.mMessage = aMessage.Clone(aCopyLength);
+    aRequest.mMessage = AsCoapMessagePtr(aMessage.Clone(aCopyLength));
     VerifyOrExit(aRequest.HasMessage(), error = kErrorNoBufs);
 
     SuccessOrExit(error = aRequest.AppendMetadataToMessage());
@@ -1669,7 +1668,7 @@ void CoapBase::PendingRequests::RetransmitRequest(const Request &aRequest)
     Message         *clone;
     Ip6::MessageInfo messageInfo;
 
-    clone = aRequest.mMessage->Clone(aRequest.mMessage->GetLength() - sizeof(Request::Metadata));
+    clone = AsCoapMessagePtr(aRequest.mMessage->Clone(aRequest.mMessage->GetLength() - sizeof(Request::Metadata)));
     VerifyOrExit(clone != nullptr, error = kErrorNoBufs);
 
     aRequest.mMetadata.CopyInfoTo(messageInfo);
@@ -1784,7 +1783,7 @@ Error CoapBase::ResponseCache::SendCachedResponse(const Msg &aRxMsg, CoapBase &a
 
     VerifyOrExit(match != nullptr, error = kErrorNotFound);
 
-    response = match->Clone(match->GetLength() - sizeof(ResponseMetadata));
+    response = AsCoapMessagePtr(match->Clone(match->GetLength() - sizeof(ResponseMetadata)));
     VerifyOrExit(response != nullptr, error = kErrorNoBufs);
 
     error = aCoapBase.Send(*response, aRxMsg.mMessageInfo);
@@ -1830,7 +1829,7 @@ void CoapBase::ResponseCache::Add(const Msg &aTxMsg, uint32_t aExchangeLifetime)
 
     MaintainCacheSize();
 
-    responseClone = aTxMsg.mMessage.Clone();
+    responseClone = AsCoapMessagePtr(aTxMsg.mMessage.Clone());
     VerifyOrExit(responseClone != nullptr);
 
     metadata.mExpireTime  = TimerMilli::GetNow() + aExchangeLifetime;

src/core/coap/coap_message.cpp

@@ -690,22 +690,6 @@ exit:
     return hasSame;
 }
 
-Message *Message::Clone(void) const { return Clone(GetLength()); }
-
-Message *Message::Clone(uint16_t aLength) const { return Clone(aLength, GetReserved()); }
-
-Message *Message::Clone(uint16_t aLength, uint16_t aReserveHeader) const
-{
-    Message *message = static_cast<Message *>(ot::Message::Clone(aLength, aReserveHeader));
-
-    VerifyOrExit(message != nullptr);
-
-    message->SetHeaderOffset(GetHeaderOffset());
-
-exit:
-    return message;
-}
-
 #if OPENTHREAD_CONFIG_COAP_API_ENABLE
 const char *Message::CodeToString(void) const
 {

src/core/coap/coap_message.hpp

@@ -826,43 +826,6 @@ public:
      */
     Error AppendPayloadMarker(void);
 
-    /**
-     * Creates a copy of the message.
-     *
-     * It allocates the new message from the same message pool as the original one and copies the entire payload. The
-     * `Type`, `SubType`, `LinkSecurity`, `Offset`, and `Priority` fields on the cloned message are also
-     * copied from the original one.
-     *
-     * @returns A pointer to the message or `nullptr` if insufficient message buffers are available.
-     */
-    Message *Clone(void) const;
-
-    /**
-     * Creates a copy of this CoAP message.
-     *
-     * It allocates the new message from the same message pool as the original one and copies @p aLength octets
-     * of the payload. The `Type`, `SubType`, `LinkSecurity`, `Offset`, and `Priority` fields on the
-     * cloned message are also copied from the original one.
-     *
-     * @param[in] aLength  Number of payload bytes to copy.
-     *
-     * @returns A pointer to the message or `nullptr` if insufficient message buffers are available.
-     */
-    Message *Clone(uint16_t aLength) const;
-
-    /**
-     * Creates a copy of the message using a given configuration.
-     *
-     * It allocates the new message from the same message pool as the original one. The `Type`, `SubType`,
-     * `LinkSecurity`, `Offset`, and `Priority` fields on the cloned message are copied from the original one.
-     *
-     * @param[in] aLength         Number of message bytes to copy.
-     * @param[in] aReserveHeader  Number of header bytes to reserve in the new cloned message.
-     *
-     * @returns A pointer to the message or `nullptr` if insufficient message buffers are available.
-     */
-    Message *Clone(uint16_t aLength, uint16_t aReserveHeader) const;
-
     /**
      * Returns a pointer to the next message after this as a `Coap::Message`.
      *