close the test gaps a QA pass found, and de-flake the flaky ones

Verify the Subsonic token hash and salt freshness, and fall back to per-album
songs when the empty-search probe fails with an I/O error instead of
aborting the whole sync.

Add a real-server sync test that mirrors navidrome and gonic into Room and
reads the rows back, plus SyncManager coverage for the success, coalescing
and mid-sync-failure paths.

Cover the sort mapping and delete/ping ViewModels, the download engine's
request flags, the queue helpers, download prefs and migration data
preservation.

Drop the leaked collectors in the ViewModel/queue tests, document the
screenshot tests as render smoke tests, bind a real search term in the
trigram plan test, and assert a positive in-flight invariant alongside the
concurrency timing checks.

app/build.gradle.kts

@@ -75,12 +75,16 @@ ksp {
     arg("room.schemaLocation", "$projectDir/schemas")
 }
 
-val integrationTestClassName = "com.subtracks.data.source.subsonic.SubsonicSourceIntegrationTest"
+val integrationTestClasses =
+    listOf(
+        "com.subtracks.data.source.subsonic.SubsonicSourceIntegrationTest",
+        "com.subtracks.data.sync.SyncServiceIntegrationTest",
+    )
 
 tasks.withType<Test>().configureEach {
     if (name.endsWith("UnitTest")) {
         filter {
-            excludeTestsMatching(integrationTestClassName)
+            integrationTestClasses.forEach { excludeTestsMatching(it) }
         }
     }
 }
@@ -93,7 +97,7 @@ tasks.register<Test>("integrationTest") {
     testClassesDirs = files(provider { unitTest.get().testClassesDirs })
     classpath = files(provider { unitTest.get().classpath })
     filter {
-        includeTestsMatching(integrationTestClassName)
+        integrationTestClasses.forEach { includeTestsMatching(it) }
     }
 }