ruff_db: use Annotation::get_message in more places

I added this accessor because tests want it, but we can also use it in
other places internally. It's a little nicer because it does the
`as_deref()` for you.

crates/ruff_db/src/diagnostic/mod.rs

@@ -138,7 +138,7 @@ impl Diagnostic {
         // in Red Knot, so we do it this way for now to match the old
         // semantics. ---AG
         self.primary_annotation()
-            .and_then(|ann| ann.message.as_deref())
+            .and_then(|ann| ann.get_message())
             .unwrap_or_default()
     }
 
@@ -170,7 +170,7 @@ impl Diagnostic {
         let main = &self.inner.message;
         let annotation = self
             .primary_annotation()
-            .and_then(|ann| ann.message.as_deref())
+            .and_then(|ann| ann.get_message())
             .unwrap_or_default();
         match (main.is_empty(), annotation.is_empty()) {
             (false, true) => ConciseMessage::MainDiagnostic(main),

crates/ruff_db/src/diagnostic/render.rs

@@ -314,7 +314,7 @@ impl<'a> ResolvedAnnotation<'a> {
             range,
             line_start,
             line_end,
-            message: ann.message.as_deref(),
+            message: ann.get_message(),
             is_primary: ann.is_primary,
         })
     }