refactor(concurrent): Introduce SequentialJob to manage service setup (#3983)

Signed-off-by: James Rich <2199651+jamesarich@users.noreply.github.com>

app/src/test/java/com/geeksville/mesh/concurrent/SequentialJobTest.kt

@@ -0,0 +1,81 @@
+/*
+ * Copyright (c) 2025 Meshtastic LLC
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package com.geeksville.mesh.concurrent
+
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.test.advanceTimeBy
+import kotlinx.coroutines.test.runTest
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+@ExperimentalCoroutinesApi
+class SequentialJobTest {
+
+    private val sequentialJob = SequentialJob()
+
+    @Test
+    fun `launch cancels previous job`() = runTest {
+        var job1Active = false
+        var job1Cancelled = false
+
+        // Launch first job
+        sequentialJob.launch(this) {
+            try {
+                job1Active = true
+                delay(1000)
+            } finally {
+                job1Cancelled = true
+            }
+        }
+
+        advanceTimeBy(100)
+        assertTrue("Job 1 should be active", job1Active)
+
+        // Launch second job
+        sequentialJob.launch(this) {
+            // Do nothing
+        }
+
+        advanceTimeBy(100)
+        assertTrue("Job 1 should be cancelled", job1Cancelled)
+    }
+
+    @Test
+    fun `cancel stops the job`() = runTest {
+        var jobActive = false
+        var jobCancelled = false
+
+        sequentialJob.launch(this) {
+            try {
+                jobActive = true
+                delay(1000)
+            } finally {
+                jobCancelled = true
+            }
+        }
+
+        advanceTimeBy(100)
+        assertTrue("Job should be active", jobActive)
+
+        sequentialJob.cancel()
+
+        advanceTimeBy(100)
+        assertTrue("Job should be cancelled", jobCancelled)
+    }
+}