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()

crates/milli/src/search/new/ranking_rule_graph/fid/mod.rs

@@ -88,10 +88,10 @@ impl RankingRuleGraphTrait for FidGraph {
         }
 
         // always lookup the max_fid if we don't already and add an artificial condition for max scoring
-        let max_weight: Option<u16> = weights_map.max_weight();
+        let max_weight_fid = weights_map.max_weight_fid();
 
-        if let Some(max_weight) = max_weight {
-            if !all_fields.contains(&max_weight) {
+        if let Some((max_fid, max_weight)) = max_weight_fid {
+            if !all_fields.contains(&max_fid) {
                 edges.push((
                     max_weight as u32 * term.term_ids.len() as u32, // TODO improve the fid score i.e. fid^10.
                     conditions_interner.insert(FidCondition {