Display an icon in the room header for rooms with shared history (#5016)

* feat: Display an icon in the room header for rooms with shared history

* fix: Apply suggestions from code review

- Simplifies `isRoomHistoryShared` expressions using type inferrence.
- Adds failure messages to unit test.

Co-authored-by: Doug <6060466+pixlwave@users.noreply.github.com>

* fix: Remove extension method in favour of field on configuration.

* fix: Distinguish between `shared` and `worldReadable` icons.

* refactor: Use `RoomHistorySharingState` enum with extension methods

---------

Co-authored-by: Doug <6060466+pixlwave@users.noreply.github.com>

ElementX/Sources/Services/Room/RoomHistorySharingState.swift

@@ -0,0 +1,15 @@
+//
+// Copyright 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.
+//
+
+/// Enumeration of the two possible cases in which history sharing under MSC4268 is enabled. These
+/// variants implicitly assume that the feature flag, `enableKeyShareOnInvite`, is set.
+enum RoomHistorySharingState: Equatable {
+    /// The feature flag is set, and the room history visibility is set to `shared`.
+    case shared
+    /// The feature flag is set, and the room history visibility is set to `world_readable`.
+    case worldReadable
+}

ElementX/Sources/Services/Room/RoomInfoProxyProtocol.swift

@@ -134,4 +134,20 @@ extension RoomInfoProxyProtocol {
         // And finally return whatever the first alternative alias is
         return alternativeAliases.first
     }
+    
+    /// If present, the state of history sharing in this room. This *does not* consider the `enableKeyShareOnInvite`
+    /// feature flag, so consumers should be careful to check the flag is true before utilising this property.
+    var historySharingState: RoomHistorySharingState? {
+        guard isEncrypted else {
+            return nil
+        }
+        return switch historyVisibility {
+        case .shared:
+            .shared
+        case .worldReadable:
+            .worldReadable
+        default:
+            nil
+        }
+    }
 }