M12f: :data tests for the 429 retry, D-040 whole-request refusals and WorkManager unique work

- SyncEngineTest: 429 stops with Retry(rate_limited) before pulling; 400, 413, 409, 422 push refusals are no
  progress, the cycle pulls, succeeds and records last_error_code, one push only.
- WorkManagerSyncWorkTest on work-testing's test WorkManager: kvitt-sync APPEND_OR_REPLACE (second request chained),
  kvitt-sync-periodic KEEP every 15 minutes, CONNECTED constraint. Adds androidx.work:work-testing (testImplementation,
  same version as work-runtime).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

android/data/build.gradle.kts

@@ -32,6 +32,7 @@ dependencies {
     implementation(libs.coil.network.ktor3)
 
     testImplementation(libs.ktor.client.mock)
+    testImplementation(libs.androidx.work.testing)
     testImplementation(libs.kotlinx.coroutines.test)
     testImplementation(libs.robolectric)
 }