fix(auto): only include unread messages in conversation notif

The MIN_CONTEXT_MESSAGES fallback (pull 3 most recent history
messages when unread < 3) was injecting already-read historical
messages into the MessagingStyle. On Android Auto, Gearhead reads
every message in the style aloud and shows them stacked on the
HUN, so old context was announced alongside the new one.

Just show the unread messages (up to MAX_HISTORY_MESSAGES). If
nothing is unread, don't post at all — we only call this from
paths that already check for fresh inbound content.

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

core/service/src/androidMain/kotlin/org/meshtastic/core/service/MeshServiceNotificationsImpl.kt

@@ -119,7 +119,6 @@ class MeshServiceNotificationsImpl(
         const val MAX_BATTERY_LEVEL = 100
         private val NOTIFICATION_LIGHT_COLOR = Color.BLUE
         private const val MAX_HISTORY_MESSAGES = 10
-        private const val MIN_CONTEXT_MESSAGES = 3
         private const val SNIPPET_LENGTH = 30
         private const val GROUP_KEY_MESSAGES = "com.geeksville.mesh.GROUP_MESSAGES"
         private const val SUMMARY_ID = 1
@@ -426,14 +425,8 @@ class MeshServiceNotificationsImpl(
                 .first()
 
         val unread = history.filter { !it.read }
-        val displayHistory =
-            if (unread.size < MIN_CONTEXT_MESSAGES) {
-                history.take(MIN_CONTEXT_MESSAGES).reversed()
-            } else {
-                unread.take(MAX_HISTORY_MESSAGES).reversed()
-            }
-
-        if (displayHistory.isEmpty()) return
+        if (unread.isEmpty()) return
+        val displayHistory = unread.take(MAX_HISTORY_MESSAGES).reversed()
 
         val notification =
             createConversationNotification(