Fix panic on diagnostic hover (#22693)

In #22620 `diagnostic_group` was modified to return results for
multibuffers, but was returning singleton buffer points. `hover_popover`
uses it to find the jump target for clicking the popup - which doesn't
seem to be working right now but that's a separate issue. Now that
`diagnostic_group` is returning values in multibuffers converting these
to anchors was crashing.

Also resolves a potential bug - if folding in multibuffers was supported
then "Go To Diagnostics" would not properly skip diagnostics from folded
regions.

Release Notes:

- N/A

crates/multi_buffer/src/multi_buffer.rs

@@ -3871,15 +3871,19 @@ impl MultiBufferSnapshot {
             .any(|excerpt| excerpt.buffer.has_diagnostics())
     }
 
-    pub fn diagnostic_group<'a, O>(
-        &'a self,
+    pub fn diagnostic_group(
+        &self,
         group_id: usize,
-    ) -> impl Iterator<Item = DiagnosticEntry<O>> + 'a
-    where
-        O: text::FromAnchor + 'a,
-    {
-        self.all_excerpts()
-            .flat_map(move |excerpt| excerpt.buffer().diagnostic_group(group_id))
+    ) -> impl Iterator<Item = DiagnosticEntry<Anchor>> + '_ {
+        self.all_excerpts().flat_map(move |excerpt| {
+            excerpt.buffer().diagnostic_group(group_id).map(
+                move |DiagnosticEntry { diagnostic, range }| DiagnosticEntry {
+                    diagnostic,
+                    range: self.anchor_in_excerpt(excerpt.id(), range.start).unwrap()
+                        ..self.anchor_in_excerpt(excerpt.id(), range.end).unwrap(),
+                },
+            )
+        })
     }
 
     pub fn diagnostics_in_range<'a, T>(