Fix inconsistency in attribute ranking rule computation

**Changes:**
The building of the Attributes ranking rule graph was comparing fieldids with weights
which doesn't make sense and may be bug prone, we are now comparing fieldids with fieldids.

**Impact:**
- search: Attribute ranking rule

crates/milli/src/fieldids_weights_map.rs

@@ -48,6 +48,11 @@ impl FieldidsWeightsMap {
         self.map.values().copied().max()
     }
 
+    /// Returns the field id with the highest weight.
+    pub fn max_weight_fid(&self) -> Option<(FieldId, Weight)> {
+        self.map.iter().max_by_key(|(_, weight)| *weight).map(|(fid, weight)| (*fid, *weight))
+    }
+
     /// Return an iterator visiting all field ids in arbitrary order.
     pub fn ids(&self) -> impl Iterator<Item = FieldId> + '_ {
         self.map.keys().copied()