Consolidate attachment saving logic into unified AttachmentSaver class.

Introduces `AttachmentSaver` to centralize all of the steps needed to save message attachments to the device storage. It handles the entire workflow including: 
- Showing the save to storage warning/confirmation dialog.
- Requesting `WRITE_EXTERNAL_STORAGE` permission.
- Showing/dismissing media save progress.

Goals of this new class:
- Make it easy to save media attachments anywhere with just a few lines of code (and easier to replace the deprecated `SaveAttachmentTask`).
- Ensure all of the necessary steps are consistently performed at each usage site (which wasn't the case before).
- Make it easier to unit test the save attachment logic.

app/src/main/java/org/thoughtcrime/securesms/util/DialogExtensions.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2025 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.util
+
+import androidx.appcompat.app.AlertDialog
+import kotlinx.coroutines.suspendCancellableCoroutine
+import kotlin.coroutines.resume
+
+/**
+ * Shows a dialog and suspends until user interaction, returning the [AlertDialogResult].
+ *
+ * Note: this method will overwrite any existing dialog button click listeners or cancellation listener.
+ */
+suspend fun AlertDialog.awaitResult(
+  positiveButtonTextId: Int? = null,
+  negativeButtonTextId: Int? = null,
+  neutralButtonTextId: Int? = null
+) = awaitResult(
+  positiveButtonText = positiveButtonTextId?.let(context::getString),
+  negativeButtonText = negativeButtonTextId?.let(context::getString),
+  neutralButtonText = neutralButtonTextId?.let(context::getString)
+)
+
+/**
+ * Shows a dialog and suspends until user interaction, returning the [AlertDialogResult].
+ *
+ * Note: this method will overwrite any existing dialog button click listeners or cancellation listener.
+ */
+suspend fun AlertDialog.awaitResult(
+  positiveButtonText: String? = null,
+  negativeButtonText: String? = null,
+  neutralButtonText: String? = null
+) = suspendCancellableCoroutine { continuation ->
+
+  positiveButtonText?.let { text -> setButton(AlertDialog.BUTTON_POSITIVE, text) { _, _ -> continuation.resume(AlertDialogResult.POSITIVE) } }
+  negativeButtonText?.let { text -> setButton(AlertDialog.BUTTON_NEGATIVE, text) { _, _ -> continuation.resume(AlertDialogResult.NEGATIVE) } }
+  neutralButtonText?.let { text -> setButton(AlertDialog.BUTTON_NEUTRAL, text) { _, _ -> continuation.resume(AlertDialogResult.NEUTRAL) } }
+
+  setOnCancelListener { continuation.resume(AlertDialogResult.CANCELED) }
+  continuation.invokeOnCancellation { dismiss() }
+
+  show()
+}
+
+enum class AlertDialogResult {
+  POSITIVE,
+  NEGATIVE,
+  NEUTRAL,
+  CANCELED
+}

app/src/main/java/org/thoughtcrime/securesms/util/SaveAttachmentUtil.kt

@@ -16,19 +16,13 @@ import android.os.Build
 import android.os.Environment
 import android.provider.MediaStore
 import android.webkit.MimeTypeMap
-import android.widget.CheckBox
-import androidx.appcompat.app.AlertDialog
 import androidx.core.content.contentValuesOf
-import com.google.android.material.dialog.MaterialAlertDialogBuilder
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.withContext
 import org.signal.core.util.StreamUtil
 import org.signal.core.util.logging.Log
-import org.signal.core.util.orNull
 import org.thoughtcrime.securesms.R
-import org.thoughtcrime.securesms.database.model.MmsMessageRecord
 import org.thoughtcrime.securesms.dependencies.AppDependencies
-import org.thoughtcrime.securesms.keyvalue.SignalStore
 import org.thoughtcrime.securesms.mms.PartAuthority
 import java.io.File
 import java.io.FileOutputStream
@@ -50,37 +44,8 @@ private typealias BatchOperationNameCache = HashMap<Uri, HashSet<String>>
  * a progress dialog and is not backed by an async task.
  */
 object SaveAttachmentUtil {
-
   private val TAG = Log.tag(SaveAttachmentUtil::class.java)
 
-  fun showWarningDialogIfNecessary(context: Context, count: Int, onSave: () -> Unit) {
-    if (SignalStore.uiHints.hasDismissedSaveStorageWarning()) {
-      onSave()
-    } else {
-      MaterialAlertDialogBuilder(context)
-        .setView(R.layout.dialog_save_attachment)
-        .setTitle(R.string.ConversationFragment__save_to_phone)
-        .setCancelable(true)
-        .setMessage(context.resources.getQuantityString(R.plurals.ConversationFragment__this_media_will_be_saved, count, count))
-        .setPositiveButton(R.string.save) { dialog, _ ->
-          val checkbox = (dialog as AlertDialog).findViewById<CheckBox>(R.id.checkbox)!!
-          if (checkbox.isChecked) {
-            SignalStore.uiHints.markDismissedSaveStorageWarning()
-          }
-          onSave()
-        }
-        .setNegativeButton(android.R.string.cancel, null)
-        .show()
-    }
-  }
-
-  fun getAttachmentsForRecord(record: MmsMessageRecord): Set<SaveAttachment> {
-    return record.slideDeck.slides
-      .filter { it.uri != null && (it.hasImage() || it.hasVideo() || it.hasAudio() || it.hasDocument()) }
-      .map { SaveAttachment(it.uri!!, it.contentType, record.dateSent, it.fileName.orNull()) }
-      .toSet()
-  }
-
   suspend fun saveAttachments(attachments: Set<SaveAttachment>): SaveAttachmentsResult {
     check(attachments.isNotEmpty()) { "must pass in at least one attachment" }