Expose a global search service from the client

Wrap the SDK's SearchService in a SearchServiceProxy that publishes typed
message results and paginates on demand, expose it on the ClientProxy, and
enable the search index store on the session. Also opens up the timeline item
factory's message content builder so the proxy can reuse it.

ElementX/Sources/Services/Search/SearchServiceProxyProtocol.swift

@@ -0,0 +1,31 @@
+// Copyright 2025 Element Creations Ltd.
+// Copyright 2025 New Vector Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+// Please see LICENSE files in the repository root for full details.
+
+import Combine
+import Foundation
+import MatrixRustSDK
+
+enum SearchServiceProxyError: Error {
+    case sdkError(Error)
+}
+
+// sourcery: AutoMockable
+protocol SearchServiceProxyProtocol {
+    var resultsPublisher: CurrentValuePublisher<[SearchServiceResult], Never> { get }
+
+    func setQuery(_ query: String) async -> Result<Void, SearchServiceProxyError>
+
+    /// Loads the next page of results. No-ops if a page is already loading or the end has been reached.
+    func paginate() async
+}
+
+struct SearchServiceResult {
+    let roomID: String
+    let eventID: String
+    let sender: TimelineItemSender
+    let content: TimelineEventContent
+    let timestamp: Date
+}