refactor: merge MeshActionHandler into DirectRadioControllerImpl (SDK-aligned)

Eliminate the AIDL-era intermediate handler layer by absorbing all
MeshActionHandler business logic directly into DirectRadioControllerImpl,
following the meshtastic-sdk's AdminApiImpl pattern.

Changes:
- Delete MeshActionHandler interface and MeshActionHandlerImpl
- DirectRadioControllerImpl now builds AdminMessage protos directly
  with no ByteArray encode/decode boundaries (typed protos all the way)
- Remove actionHandler from MeshRouter interface and impl
- Update EnsureRemoteAdminSessionUseCase to use RadioController directly
- Update DI modules (Android annotations + Desktop Koin DSL)
- Rewrite DirectRadioControllerImplTest for the merged architecture
- Update MeshRouterImplTest and MeshServiceOrchestratorTest

Architecture:
  Before: ViewModel → RadioController → Handler → CommandSender
  After:  ViewModel → RadioController → CommandSender

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jamesarich

core/data/src/commonMain/kotlin/org/meshtastic/core/data/manager/MeshActionHandlerImpl.kt

@@ -17,7 +17,6 @@
 package org.meshtastic.core.data.manager
 
 import org.koin.core.annotation.Single
-import org.meshtastic.core.repository.MeshActionHandler
 import org.meshtastic.core.repository.MeshConfigFlowManager
 import org.meshtastic.core.repository.MeshConfigHandler
 import org.meshtastic.core.repository.MeshDataHandler
@@ -37,7 +36,6 @@ class MeshRouterImpl(
     private val neighborInfoHandlerLazy: Lazy<NeighborInfoHandler>,
     private val configFlowManagerLazy: Lazy<MeshConfigFlowManager>,
     private val mqttManagerLazy: Lazy<MqttManager>,
-    private val actionHandlerLazy: Lazy<MeshActionHandler>,
     private val xmodemManagerLazy: Lazy<XModemManager>,
 ) : MeshRouter {
     override val dataHandler: MeshDataHandler
@@ -58,9 +56,6 @@ class MeshRouterImpl(
     override val mqttManager: MqttManager
         get() = mqttManagerLazy.value
 
-    override val actionHandler: MeshActionHandler
-        get() = actionHandlerLazy.value
-
     override val xmodemManager: XModemManager
         get() = xmodemManagerLazy.value
 }