Cover both MediaStore save paths with a fake provider

DefaultFileSave had no test at all and AndroidLocalMediaActions only exercised the
failing path, so neither the uniquifying retry nor the new IOException on a null
insert was ever executed.

FakeMediaStoreContentProvider stands in for the media authority: it records the
display names an insert is attempted with, keeps whatever is copied into it, and can
either collide a name the way MediaStore does past 32 files or refuse to create
anything. Both call sites now go through it for the plain save, the retry and the
rejected insert.

Its name cannot follow the Fake<Interface> convention because it extends the
framework ContentProvider, so it joins the existing exceptions in
KonsistClassNameTest next to FakeFileSystem and FakeKeyStore.

tests/testutils/src/main/kotlin/io/element/android/tests/testutils/fake/FakeMediaStoreContentProvider.kt

@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.tests.testutils.fake
+
+import android.content.ContentProvider
+import android.content.ContentValues
+import android.content.pm.ProviderInfo
+import android.database.Cursor
+import android.net.Uri
+import android.os.ParcelFileDescriptor
+import android.provider.MediaStore
+import androidx.test.core.app.ApplicationProvider
+import org.robolectric.shadows.ShadowContentResolver
+import java.io.File
+import java.nio.file.Files
+
+/**
+ * Stands in for the MediaStore provider: it records the display names an insert is attempted with,
+ * writes what is copied into it to a temporary directory, and can make any display name collide the
+ * way MediaStore does once a directory already holds 32 files with the same name.
+ */
+class FakeMediaStoreContentProvider : ContentProvider() {
+    private val directory: File = Files.createTempDirectory("fake-media-store").toFile()
+
+    val insertedDisplayNames = mutableListOf<String>()
+
+    var conflictingDisplayNames: Set<String> = emptySet()
+
+    /** When true, an insert reports that it created nothing, the way a rejected insert does. */
+    var returnsNoUriOnInsert: Boolean = false
+
+    fun savedFileNames(): List<String> = directory.list().orEmpty().toList()
+
+    fun savedContent(displayName: String): String = File(directory, displayName).readText()
+
+    override fun onCreate() = true
+
+    override fun insert(uri: Uri, values: ContentValues?): Uri? {
+        val displayName = values?.getAsString(MediaStore.MediaColumns.DISPLAY_NAME).orEmpty()
+        insertedDisplayNames += displayName
+        check(displayName !in conflictingDisplayNames) { "Failed to build unique file" }
+        return if (returnsNoUriOnInsert) null else uri.buildUpon().appendPath(displayName).build()
+    }
+
+    override fun openFile(uri: Uri, mode: String): ParcelFileDescriptor {
+        val file = File(directory, uri.lastPathSegment.orEmpty())
+        return ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_CREATE or ParcelFileDescriptor.MODE_READ_WRITE)
+    }
+
+    override fun query(uri: Uri, projection: Array<out String>?, selection: String?, selectionArgs: Array<out String>?, sortOrder: String?): Cursor? = null
+
+    override fun getType(uri: Uri): String? = null
+
+    override fun update(uri: Uri, values: ContentValues?, selection: String?, selectionArgs: Array<out String>?) = 0
+
+    override fun delete(uri: Uri, selection: String?, selectionArgs: Array<out String>?) = 0
+}
+
+fun registerFakeMediaStoreContentProvider(): FakeMediaStoreContentProvider {
+    return FakeMediaStoreContentProvider().apply {
+        attachInfo(
+            ApplicationProvider.getApplicationContext(),
+            ProviderInfo().apply { authority = MediaStore.AUTHORITY }
+        )
+        ShadowContentResolver.registerProviderInternal(MediaStore.AUTHORITY, this)
+    }
+}