[commissioner] simplify energy list parsing in report handler (#12666)

This commit updates `Commissioner::HandleTmf<kUriEnergyReport>()` to
read the energy list data directly into a local array instead of
using a dedicated TLV class.

The report handler now uses `Tlv::FindTlvValueOffsetRange()` to locate
the TLV value, which works correctly whether the TLV is encoded as a
standard or extended TLV. With this change, the `EnergyListTlv` class
definition is replaced with a simple typedef to `TlvInfo`.

src/core/meshcop/commissioner.cpp

@@ -1070,8 +1070,9 @@ exit:
 
 template <> void Commissioner::HandleTmf<kUriEnergyReport>(Coap::Msg &aMsg)
 {
-    uint32_t      mask;
-    EnergyListTlv energyListTlv;
+    uint32_t    mask;
+    OffsetRange valueOffsetRange;
+    uint8_t     results[kMaxEnergyScanResults];
 
     VerifyOrExit(aMsg.IsConfirmable());
 
@@ -1079,9 +1080,11 @@ template <> void Commissioner::HandleTmf<kUriEnergyReport>(Coap::Msg &aMsg)
 
     SuccessOrExit(ChannelMaskTlv::FindIn(aMsg.mMessage, mask));
 
-    SuccessOrExit(Tlv::FindTlv(aMsg.mMessage, Tlv::kEnergyList, sizeof(energyListTlv), energyListTlv));
+    SuccessOrExit(Tlv::FindTlvValueOffsetRange(aMsg.mMessage, Tlv::kEnergyList, valueOffsetRange));
+    valueOffsetRange.ShrinkLength(sizeof(results));
+    aMsg.mMessage.ReadBytes(valueOffsetRange, results);
 
-    mEnergyReportCallback.InvokeIfSet(mask, energyListTlv.GetEnergyList(), energyListTlv.GetEnergyListLength());
+    mEnergyReportCallback.InvokeIfSet(mask, results, static_cast<uint8_t>(valueOffsetRange.GetLength()));
 
     SuccessOrExit(Get<Tmf::Agent>().SendAckResponse(aMsg));
 

src/core/meshcop/commissioner.hpp

@@ -382,6 +382,8 @@ private:
     static constexpr uint32_t kJoinerSessionTimeoutMillis =
         1000 * OPENTHREAD_CONFIG_COMMISSIONER_JOINER_SESSION_TIMEOUT; // Expiration time for active Joiner session
 
+    static constexpr uint8_t kMaxEnergyScanResults = OPENTHREAD_CONFIG_TMF_ENERGY_SCAN_MAX_RESULTS;
+
     enum ResignMode : uint8_t
     {
         kSendKeepAliveToResign,

src/core/meshcop/meshcop_tlvs.hpp

@@ -724,48 +724,9 @@ private:
 } OT_TOOL_PACKED_BEGIN;
 
 /**
- * Implements Energy List TLV generation and parsing.
+ * Defines Energy List TLV constants and types.
  */
-OT_TOOL_PACKED_BEGIN
-class EnergyListTlv : public Tlv, public TlvInfo<Tlv::kEnergyList>
-{
-public:
-    /**
-     * Initializes the TLV.
-     */
-    void Init(void)
-    {
-        SetType(kEnergyList);
-        SetLength(sizeof(*this) - sizeof(Tlv));
-    }
-
-    /**
-     * Indicates whether or not the TLV appears to be well-formed.
-     *
-     * @retval TRUE   If the TLV appears to be well-formed.
-     * @retval FALSE  If the TLV does not appear to be well-formed.
-     */
-    bool IsValid(void) const { return true; }
-
-    /**
-     * Returns a pointer to the start of energy measurement list.
-     *
-     * @returns A pointer to the start start of energy energy measurement list.
-     */
-    const uint8_t *GetEnergyList(void) const { return mEnergyList; }
-
-    /**
-     * Returns the length of energy measurement list.
-     *
-     * @returns The length of energy measurement list.
-     */
-    uint8_t GetEnergyListLength(void) const { return Min(kMaxListLength, GetLength()); }
-
-private:
-    static constexpr uint8_t kMaxListLength = OPENTHREAD_CONFIG_TMF_ENERGY_SCAN_MAX_RESULTS;
-
-    uint8_t mEnergyList[kMaxListLength];
-} OT_TOOL_PACKED_END;
+typedef TlvInfo<Tlv::kEnergyList> EnergyListTlv;
 
 /**
  * Defines Provisioning TLV constants and types.