fix(sidecar): use ephemeral host port for AndroidDriver

Hard-coded port 7001 caused TcpForwarder.start() to fail with
TimeoutException when a previous sidecar process held the port open.
Pick a free port via ServerSocket(0) instead.

sidecar/src/main/kotlin/dev/sanderling/sidecar/DriverBackend.kt

@@ -382,7 +382,8 @@ class MaestroDriverBackend(private val serial: String?) : DriverBackend {
 
     init {
         dadb = buildDadb(serial)
-        driver = maestro.drivers.AndroidDriver(dadb, 7001, "localhost")
+        val hostPort = java.net.ServerSocket(0).use { it.localPort }
+        driver = maestro.drivers.AndroidDriver(dadb, hostPort, "localhost")
         driver.open()
     }