[sub-mac] rename CslReceiver::HandleTimer() to ScheduleSampleWindow() (#13615)

The method `SubMac::CslReceiver::HandleTimer()` determines the current
CSL sample window, schedules radio reception, advances the sample time
to the next period, and re-arms the timer.

Besides timer expiration, this method is also called directly from:
- `SetParams()` to schedule the initial sample window when CSL is enabled.
- `RestartTimerAfterSyncUpdate()` to re-evaluate and reschedule the window
  following a sync update.

Calling `HandleTimer()` directly from these non-timer contexts was
semantically misleading. This commit renames the method to
`ScheduleSampleWindow()`, which accurately reflects its role as
advancing and scheduling the CSL sample window, with `HandleTimer()`
simply delegating to `ScheduleSampleWindow()`.

src/core/mac/sub_mac.hpp

@@ -632,7 +632,7 @@ private:
         void SetParams(uint16_t aPeriod, uint8_t aChannel, ShortAddress aShortAddr, const ExtAddress &aExtAddr);
         void ProcessTxDone(const TxFrame::ParseInfo &aFrameInfo, RxFrame *aAckFrame);
         void ProcessRxFrame(const RxFrame &aFrame);
-        void HandleTimer(void);
+        void HandleTimer(void) { ScheduleSampleWindow(); }
 
         const CslAccuracy &GetParentAccuracy(void) const { return mParentAccuracy; }
         void               SetParentAccuracy(const CslAccuracy &aCslAccuracy) { mParentAccuracy = aCslAccuracy; }
@@ -650,6 +650,7 @@ private:
 
         void     RestartTimerAfterSyncUpdate(void);
         void     SetLastSyncToNow(void);
+        void     ScheduleSampleWindow(void);
         void     DetermineWindow(const Radio::SyncedTime &aSampleTime, Window &aWindow) const;
         uint32_t DetermineClockDrift(uint32_t aIntervalUs) const;
         bool     IsEnabled(void) const { return mPeriod > 0; }

src/core/mac/sub_mac_csl_receiver.cpp

@@ -72,12 +72,12 @@ void SubMac::CslReceiver::RestartTimerAfterSyncUpdate(void)
 
         mTimer.Stop();
 
-        // Rewind sample times by one period. HandleTimer() will add this
-        // period back, effectively re-evaluating the current CSL period's
+        // Rewind sample times by one period. `ScheduleSampleWindow()` will add
+        // this period back, effectively re-evaluating the current CSL period's
         // schedule using the updated mLastSync.
         mSampleTime -= periodUs;
 
-        HandleTimer();
+        ScheduleSampleWindow();
     }
 }
 
@@ -146,7 +146,7 @@ void SubMac::CslReceiver::SetParams(uint16_t          aPeriod,
 #else
         mLastSync = mSampleTime.GetAsTime64();
 #endif
-        HandleTimer();
+        ScheduleSampleWindow();
     }
     else
     {
@@ -157,7 +157,7 @@ exit:
     return;
 }
 
-void SubMac::CslReceiver::HandleTimer(void)
+void SubMac::CslReceiver::ScheduleSampleWindow(void)
 {
     Window window;