Add Duck.ai chat download killswitch (#5838)

Task/Issue URL:
https://app.asana.com/0/488551667048375/1209839796813035/f

### Description

- Add a killswitch around the chat download functionality.

### Steps to test this PR

- [x] Go to Duck.ai and send a prompt
- [x] Go to recent chats and download a chat
- [x] Verify that the chat is downloaded
- [x] Go to feature flag inventory and disable `aiChatDownload`
- [x] Go to recent chats and download a chat
- [x] Verify that the chat is not downloaded

Author: joshliebe

duckchat/duckchat-impl/src/main/java/com/duckduckgo/duckchat/impl/AIChatDownloadFeature.kt

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2025 DuckDuckGo
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.duckduckgo.duckchat.impl
+
+import com.duckduckgo.anvil.annotations.ContributesRemoteFeature
+import com.duckduckgo.di.scopes.AppScope
+import com.duckduckgo.feature.toggles.api.Toggle
+
+@ContributesRemoteFeature(
+    scope = AppScope::class,
+    featureName = "aiChatDownload",
+)
+
+interface AIChatDownloadFeature {
+
+    @Toggle.DefaultValue(true)
+    fun self(): Toggle
+}

duckchat/duckchat-impl/src/main/java/com/duckduckgo/duckchat/impl/DuckChatWebViewActivity.kt

@@ -107,6 +107,9 @@ class DuckChatWebViewActivity : DuckDuckGoActivity(), DownloadConfirmationDialog
     @Inject
     lateinit var duckChat: DuckChatInternal
 
+    @Inject
+    lateinit var aiChatDownloadFeature: AIChatDownloadFeature
+
     private val binding: ActivityDuckChatWebviewBinding by viewBinding()
 
     private var pendingFileDownload: PendingFileDownload? = null
@@ -163,7 +166,11 @@ class DuckChatWebViewActivity : DuckDuckGoActivity(), DownloadConfirmationDialog
             }
 
             it.setDownloadListener { url, _, contentDisposition, mimeType, _ ->
-                requestFileDownload(url, contentDisposition, mimeType)
+                appCoroutineScope.launch(dispatcherProvider.io()) {
+                    if (aiChatDownloadFeature.self().isEnabled()) {
+                        requestFileDownload(url, contentDisposition, mimeType)
+                    }
+                }
             }
 
             contentScopeScripts.register(