Unify Message variants (#18051)

## Summary

This PR unifies the ruff `Message` enum variants for syntax errors and
rule violations into a single `Message` struct consisting of a shared
`db::Diagnostic` and some additional, optional fields used for some rule
violations.

This version of `Message` is nearly a drop-in replacement for
`ruff_diagnostics::Diagnostic`, which is the next step I have in mind
for the refactor.

I think this is also a useful checkpoint because we could possibly add
some of these optional fields to the new `Diagnostic` type. I think
we've previously discussed wanting support for `Fix`es, but the other
fields seem less relevant, so we may just need to preserve the `Message`
wrapper for a bit longer.

## Test plan

Existing tests

---------

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_linter/src/codes.rs

@@ -10,7 +10,7 @@ use crate::registry::{AsRule, Linter};
 use crate::rule_selector::is_single_rule_selector;
 use crate::rules;
 
-#[derive(PartialEq, Eq, PartialOrd, Ord)]
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 pub struct NoqaCode(&'static str, &'static str);
 
 impl NoqaCode {
@@ -46,6 +46,15 @@ impl PartialEq<&str> for NoqaCode {
     }
 }
 
+impl serde::Serialize for NoqaCode {
+    fn serialize<S>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        serializer.serialize_str(&self.to_string())
+    }
+}
+
 #[derive(Debug, Copy, Clone)]
 pub enum RuleGroup {
     /// The rule is stable.