[mac] enforce single initialization lifecycle for TxFrames (#13264)

This commit refactors `Mac::Links` and `Mac` frame preparation to ensure
`TxFrames` buffers are initialized (`Clear()`) exactly once per TX cycle.

Specifically, this commit:
- Replaces `Links::GetTxFrames()` with `Links::InitTxFrames()`, which
  encapsulates calling `Clear()` on `mTxFrames` prior to returning its
  reference. This prevents callers from retrieving uninitialized or dirty
  transmission buffers from a previous radio cycle.
- Updates `PrepareBeaconRequest()` and `PrepareBeacon()` to accept the
  initialized `TxFrames &` reference directly as a parameter, making the
  data flow explicit.
- Replaces non-const `GetTxFrames()` calls in `HandleTransmitDone()` with
  `GetTxFramesRequiredRadioTypes()`, enforcing const-correctness during
  TX completion handling.

src/core/mac/mac.cpp

@@ -703,9 +703,9 @@ void Mac::FinishOperation(void)
     mOperation = kOperationIdle;
 }
 
-TxFrame *Mac::PrepareBeaconRequest(void)
+TxFrame *Mac::PrepareBeaconRequest(TxFrames &aTxFrames)
 {
-    TxFrame      &frame = mLinks.GetTxFrames().GetBroadcastTxFrame();
+    TxFrame      &frame = aTxFrames.GetBroadcastTxFrame();
     TxFrame::Info frameInfo;
 
     frameInfo.mAddrs.mSource.SetNone();
@@ -723,7 +723,7 @@ TxFrame *Mac::PrepareBeaconRequest(void)
     return &frame;
 }
 
-TxFrame *Mac::PrepareBeacon(void)
+TxFrame *Mac::PrepareBeacon(TxFrames &aTxFrames)
 {
     TxFrame      *frame;
     TxFrame::Info frameInfo;
@@ -735,10 +735,10 @@ TxFrame *Mac::PrepareBeacon(void)
 
 #if OPENTHREAD_CONFIG_MULTI_RADIO
     OT_ASSERT(!mTxBeaconRadioLinks.IsEmpty());
-    frame = &mLinks.GetTxFrames().GetTxFrame(mTxBeaconRadioLinks);
+    frame = &aTxFrames.GetTxFrame(mTxBeaconRadioLinks);
     mTxBeaconRadioLinks.Clear();
 #else
-    frame = &mLinks.GetTxFrames().GetBroadcastTxFrame();
+    frame = &aTxFrames.GetBroadcastTxFrame();
 #endif
 
     frameInfo.mAddrs.mSource.SetExtended(GetExtAddress());
@@ -917,11 +917,9 @@ exit:
 void Mac::BeginTransmit(void)
 {
     TxFrame  *frame    = nullptr;
-    TxFrames &txFrames = mLinks.GetTxFrames();
+    TxFrames &txFrames = mLinks.InitTxFrames();
     Address   dstAddr;
 
-    txFrames.Clear();
-
 #if OPENTHREAD_CONFIG_MULTI_RADIO
     mTxPendingRadioLinks.Clear();
     mTxError = kErrorAbort;
@@ -933,7 +931,7 @@ void Mac::BeginTransmit(void)
     {
     case kOperationActiveScan:
         mLinks.SetPanId(kPanIdBroadcast);
-        frame = PrepareBeaconRequest();
+        frame = PrepareBeaconRequest(txFrames);
         VerifyOrExit(frame != nullptr);
         frame->SetChannel(mScanChannel);
         frame->SetSequence(0);
@@ -942,7 +940,7 @@ void Mac::BeginTransmit(void)
         break;
 
     case kOperationTransmitBeacon:
-        frame = PrepareBeacon();
+        frame = PrepareBeacon(txFrames);
         VerifyOrExit(frame != nullptr);
         frame->SetChannel(mRadioChannel);
         frame->SetSequence(mBeaconSequence++);
@@ -1319,7 +1317,7 @@ void Mac::HandleTransmitDone(TxFrame &aFrame, RxFrame *aAckFrame, Error aError)
     if (!aFrame.IsEmpty())
     {
         RadioType  radio          = aFrame.GetRadioType();
-        RadioTypes requiredRadios = mLinks.GetTxFrames().GetRequiredRadioTypes();
+        RadioTypes requiredRadios = mLinks.GetTxFramesRequiredRadioTypes();
 
         Get<RadioSelector>().UpdateOnSendDone(aFrame, aError);
 

src/core/mac/mac.hpp

@@ -832,8 +832,8 @@ private:
     void     StartOperation(Operation aOperation);
     void     FinishOperation(void);
     void     PerformNextOperation(void);
-    TxFrame *PrepareBeaconRequest(void);
-    TxFrame *PrepareBeacon(void);
+    TxFrame *PrepareBeaconRequest(TxFrames &aTxFrames);
+    TxFrame *PrepareBeacon(TxFrames &aTxFrames);
     bool     ShouldSendBeacon(void) const;
     bool     IsJoinable(void) const;
     void     BeginTransmit(void);

src/core/mac/mac_links.hpp

@@ -517,18 +517,22 @@ public:
     }
 
     /**
-     * Gets the radio transmit frames.
+     * Initializes and retrieves the radio transmit frames `TxFrames`.
      *
      * @returns The transmit frames.
      */
-    TxFrames &GetTxFrames(void) { return mTxFrames; }
+    TxFrames &InitTxFrames(void)
+    {
+        mTxFrames.Clear();
+        return mTxFrames;
+    }
 
 #if !OPENTHREAD_CONFIG_MULTI_RADIO
 
     /**
      * Sends a prepared frame.
      *
-     * The prepared frame is from `GetTxFrames()`. This method is available only in single radio link mode.
+     * The prepared frame is from `InitTxFrames()`. This method is available only in single radio link mode.
      */
     void Send(void)
     {
@@ -545,13 +549,24 @@ public:
     /**
      * Sends prepared frames over a given set of radio links.
      *
-     * The prepared frame must be from `GetTxFrames()`. This method is available only in multi radio link mode.
+     * The prepared frame must be from `InitTxFrames()`. This method is available only in multi radio link mode.
      *
      * @param[in] aFrame       A reference to a prepared frame.
      * @param[in] aRadioTypes  A set of radio types to send on.
      */
     void Send(TxFrame &aFrame, RadioTypes aRadioTypes);
 
+    /**
+     * Gets the required radio types (`GetRequiredRadioTypes()`) from `TxFrames`.
+     *
+     * This set specifies the radio links for which we expect the frame tx to be successful to consider the overall tx
+     * successful. If the set is empty, successful tx over any radio link is sufficient for overall tx to be considered
+     * successful. The required radio type set is expected to be a subset of selected radio types.
+     *
+     * @returns The required radio types.
+     */
+    RadioTypes GetTxFramesRequiredRadioTypes(void) const { return mTxFrames.GetRequiredRadioTypes(); }
+
 #endif // !OPENTHREAD_CONFIG_MULTI_RADIO
 
     /**