[sub-mac] simplify SubMac::Sleep() implementation (#13429)

This commit simplifies `SubMac::Sleep()` by inlining the logic
previously in `RadioSleep()` and `ShouldHandleTransitionToSleep()`,
allowing the removal of those private helper methods.

It also clarifies via comment why `Radio::Sleep()` is invoked when
`mRxOnWhenIdle` is true even if `kCapRxOnWhenIdle` is supported
(to support radio validation and test scenarios where the radio is
forced to sleep).

src/core/mac/sub_mac.cpp

@@ -215,21 +215,17 @@ Error SubMac::Sleep(void)
     if (IsRadioSampleEnabled())
     {
         RadioSample();
+        ExitNow();
     }
-    else
 #endif
-    {
-        error = RadioSleep();
-    }
 
-    return error;
-}
+    // Even if the radio platform supports `kCapRxOnWhenIdle`, when
+    // `SubMac::Sleep()` is explicitly called while `mRxOnWhenIdle`
+    // is true, we still call `Radio::Sleep()`. This supports radio
+    // validation and test scenarios where the radio is being forced
+    // to sleep.
 
-Error SubMac::RadioSleep(void)
-{
-    Error error = kErrorNone;
-
-    if (ShouldHandleTransitionToSleep())
+    if (!RadioSupports(kCapRxOnWhenIdle) || mRxOnWhenIdle)
     {
         SuccessOrExit(error = Get<Radio::Radio>().Sleep());
     }
@@ -237,7 +233,7 @@ Error SubMac::RadioSleep(void)
     SetState(kStateSleep);
 
 exit:
-    LogWarnOnError(error, "RadioSleep()");
+    LogWarnOnError(error, "Sleep()");
     return error;
 }
 
@@ -891,8 +887,6 @@ exit:
     return shouldHandle;
 }
 
-bool SubMac::ShouldHandleTransitionToSleep(void) const { return (mRxOnWhenIdle || !RadioSupports(kCapRxOnWhenIdle)); }
-
 void SubMac::SetState(State aState)
 {
     if (mState != aState)

src/core/mac/sub_mac.hpp

@@ -581,7 +581,6 @@ private:
     bool ShouldHandleEnergyScan(void) const { return ShouldHandle(kCapEnergyScan); }
     bool ShouldHandleTransmitTargetTime(void) const { return ShouldHandle(kCapTransmitTiming); }
     bool ShouldHandleCsmaBackOff(void) const;
-    bool ShouldHandleTransitionToSleep(void) const;
 
     void ProcessTransmitSecurity(void);
     void ReprocessSecurityForRetx(TxFrame &aFrame);
@@ -600,8 +599,6 @@ private:
     void HandleEnergyScanDone(int8_t aMaxRssi);
     void HandleTimer(void);
 
-    Error RadioSleep(void);
-
     void               SetState(State aState);
     static const char *StateToString(State aState);