[ty] Don't create a related diagnostic for the primary annotation of sub-diagnostics (#21845)

crates/ruff_db/src/diagnostic/mod.rs

@@ -888,6 +888,10 @@ impl Annotation {
     pub fn hide_snippet(&mut self, yes: bool) {
         self.hide_snippet = yes;
     }
+
+    pub fn is_primary(&self) -> bool {
+        self.is_primary
+    }
 }
 
 /// Tags that can be associated with an annotation.

crates/ty_server/src/server/api/diagnostics.rs

@@ -349,6 +349,7 @@ pub(super) fn to_lsp_diagnostic(
             sub_diagnostic
                 .annotations()
                 .iter()
+                .filter(|annotation| !annotation.is_primary())
                 .filter_map(|annotation| {
                     annotation_to_related_information(db, annotation, encoding)
                 }),