add type annotation to silence rust-analyzer (#23806)

there's a bug in rust-analyzer causing a false positive error on this line.
i tried for a while to find a smaller repro but couldn't figure one out.
to avoid this popping up and distracting us, add a type annotation.

GitOrigin-RevId: 10d3dd838b7991686e53892eeb4de8d9b42d14ed

Author: ldanilek

crates/database/src/vector_index_worker/flusher.rs

@@ -333,7 +333,9 @@ impl<RT: Runtime> VectorIndexFlusher<RT> {
         } else {
             None
         };
-        let new_segment_id = new_segment.as_ref().map(|segment| segment.id.clone());
+        let new_segment_id = new_segment
+            .as_ref()
+            .map(|segment: &FragmentedVectorSegment| segment.id.clone());
         let vectors_in_new_segment = new_segment.as_ref().map(|segment| segment.num_vectors);
 
         let new_and_updated_parts = if let Some(new_segment) = new_segment {