[mdns] change SetEnabled() to be idempotent (#11690)

This commit changes the `SetEnabled()` API to return `kErrorNone` when
the component is already in the requested state (enabled or
disabled). Previously, the method would return `kErrorAlready` in
this scenario. Making this API idempotent simplifies caller logic, as
they no longer need to handle the `kErrorAlready` case.

include/openthread/instance.h

@@ -52,7 +52,7 @@ extern "C" {
  *
  * @note This number versions both OpenThread platform and user APIs.
  */
-#define OPENTHREAD_API_VERSION (518)
+#define OPENTHREAD_API_VERSION (519)
 
 /**
  * @addtogroup api-instance

include/openthread/mdns.h

@@ -170,7 +170,7 @@ typedef struct otMdnsLocalHostAddress
  * @param[in] aInfraIfIndex  The network interface index for mDNS operation. Value is ignored when disabling
  *
  * @retval OT_ERROR_NONE     Enabled or disabled the mDNS module successfully.
- * @retval OT_ERROR_ALREADY  mDNS is already enabled on an enable request or is already disabled on a disable request.
+ * @retval OT_ERROR_FAILED   Failed to enable/disable mDNS.
  */
 otError otMdnsSetEnabled(otInstance *aInstance, bool aEnable, uint32_t aInfraIfIndex);
 

src/core/net/mdns.cpp

@@ -119,7 +119,7 @@ Error Core::SetEnabled(bool aEnable, uint32_t aInfraIfIndex, Requester aRequeste
         mAutoEnable = false;
     }
 
-    VerifyOrExit(aEnable != mIsEnabled, error = kErrorAlready);
+    VerifyOrExit(aEnable != mIsEnabled);
 
     mIsEnabled    = aEnable;
     mInfraIfIndex = aInfraIfIndex;

src/core/net/mdns.hpp

@@ -175,7 +175,6 @@ public:
      * @param[in] aInfraIfIndex The network interface index for mDNS operation. Value is ignored when disabling.
      *
      * @retval kErrorNone     Enabled or disabled the mDNS module successfully.
-     * @retval kErrorAlready  mDNS is already enabled on an enable request, or is already disabled on a disable request.
      * @retval kErrorFailed   Failed to enable/disable mDNS.
      */
     Error SetEnabled(bool aEnable, uint32_t aInfraIfIndex)