feat(sdk): Add LinkedChunk::first_chunk.

This patch adds `LinkedChunk::first_chunk`, and exposes it to
`EventLinkedChunk` to replace the use of 2 iterators in `RoomPagination`
and `ThreadPagination`, and to remove 2 `unwrap`s.

crates/matrix-sdk-common/src/linked_chunk/mod.rs

@@ -1110,6 +1110,11 @@ impl<const CAP: usize, Item, Gap> LinkedChunk<CAP, Item, Gap> {
             .skip(position.index()))
     }
 
+    /// Return the first chunk.
+    pub fn first_chunk(&self) -> &Chunk<CAP, Item, Gap> {
+        self.links.first_chunk()
+    }
+
     /// Get a mutable reference to the `LinkedChunk` updates, aka
     /// [`ObservableUpdates`].
     ///

crates/matrix-sdk/src/event_cache/caches/event_linked_chunk.rs

@@ -177,6 +177,11 @@ impl EventLinkedChunk {
         self.chunks.chunk_identifier(predicate)
     }
 
+    /// Return the first chunk.
+    pub fn first_chunk(&self) -> &Chunk<DEFAULT_CHUNK_CAPACITY, Event, Gap> {
+        self.chunks.first_chunk()
+    }
+
     /// Iterate over the chunks, forward.
     ///
     /// The oldest chunk comes first.

crates/matrix-sdk/src/event_cache/caches/room/pagination.rs

@@ -170,8 +170,7 @@ impl PaginatedCache for Arc<RoomEventCacheInner> {
             });
         }
 
-        let prev_first_chunk =
-            state.room_linked_chunk().chunks().next().expect("a linked chunk is never empty");
+        let prev_first_chunk = state.room_linked_chunk().first_chunk();
 
         // The first chunk is not a gap, we can load its previous chunk.
         let linked_chunk_id = LinkedChunkId::Room(&state.state.room_id);

crates/matrix-sdk/src/event_cache/caches/thread/pagination.rs

@@ -112,8 +112,7 @@ impl PaginatedCache for ThreadEventCacheWrapper {
             });
         }
 
-        let prev_first_chunk =
-            state.thread_linked_chunk().chunks().next().expect("a linked chunk is never empty");
+        let prev_first_chunk = state.thread_linked_chunk().first_chunk();
 
         // The first chunk is not a gap, we can load its previous chunk.
         let linked_chunk_id = LinkedChunkId::Thread(&state.room_id, &state.thread_id);