net: pin the user agent value and test the shared client

Build the shared OkHttpClient through UserAgent.httpClient() so the unit test exercises the same client the app uses, assert the exact 'Subtracks (Android; <version>)' value rather than the property it writes, and run the test under the Android runner.

app/src/main/kotlin/com/subtracks/SubtracksApp.kt

@@ -9,12 +9,11 @@ import com.subtracks.data.prefs.UserPreferences
 import com.subtracks.data.repo.ArtworkSeedStore
 import com.subtracks.di.appModule
 import com.subtracks.ui.theme.ArtworkSeedCache
-import okhttp3.OkHttpClient
 import org.koin.core.context.startKoin
 import org.koin.core.context.stopKoin
 
 class SubtracksApp : Application() {
-    private val http = OkHttpClient.Builder().addInterceptor(UserAgent.interceptor()).build()
+    private val http = UserAgent.httpClient()
 
     @OptIn(DelicateCoilApi::class)
     override fun onCreate() {

app/src/main/kotlin/com/subtracks/UserAgent.kt

@@ -1,6 +1,7 @@
 package com.subtracks
 
 import okhttp3.Interceptor
+import okhttp3.OkHttpClient
 
 object UserAgent {
     val value: String = "Subtracks (Android; ${BuildConfig.VERSION_NAME})"
@@ -15,4 +16,6 @@ object UserAgent {
                     .build(),
             )
         }
+
+    fun httpClient(): OkHttpClient = OkHttpClient.Builder().addInterceptor(interceptor()).build()
 }