Keep the map drawing test off the network and the user's tile cache

The test assumed no tile ever arrives, but it used the default TileFetcher,
which reads the real disk cache and the OSM tile server. On CI a tile now and
then landed before the frame was rendered and covered the background the
'unreachable tile source' test asserts on. An empty temporary cache and a
source that always fails make the assumption hold by construction.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app-desktop/src/test/kotlin/kst4contest/view/compose/map/ComposeStationMapDrawingTest.kt

@@ -11,6 +11,8 @@ import org.junit.jupiter.api.Assertions.assertTrue
 import org.junit.jupiter.api.Test
 import java.awt.image.BufferedImage
 import java.io.ByteArrayInputStream
+import java.io.IOException
+import java.nio.file.Files
 import javax.imageio.ImageIO
 import kotlin.math.abs
 
@@ -42,6 +44,16 @@ class ComposeStationMapDrawingTest {
 
     private val darkMapBackground = 0x23282D
 
+    /*
+     * An empty cache and a source that always fails, so that "tiles never arrive" holds by
+     * construction. The default fetcher read the user's disk cache and the real tile
+     * server, and on CI a tile now and then made it in before the frame was rendered.
+     */
+    private val offlineTileFetcher = TileFetcher(
+        Files.createTempDirectory("kst4contest-tile-cache-test"),
+        { throw IOException("no tile source in this test") },
+    )
+
     private fun station(call: String, locator: String, lat: Double, lon: Double) =
         MapCallsignRawSnapshot(
             call, call, locator, lat, lon, "", emptyMap(),
@@ -56,7 +68,7 @@ class ComposeStationMapDrawingTest {
         val scene = ImageComposeScene(canvasWidth, canvasHeight, Density(1f)) {
             ComposeStationMap(
                 markers = markers,
-                tileFetcher = TileFetcher(),
+                tileFetcher = offlineTileFetcher,
                 mapState = MapState(zoom = zoom, centerLon = 10.0, centerLat = 51.0),
                 groupingEnabled = false,
                 selectedMarkerCallsign = null,