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_macros/src/map_codes.rs

@@ -471,13 +471,6 @@ fn register_rules<'a>(input: impl Iterator<Item = &'a Rule>) -> TokenStream {
             }
         }
 
-        impl AsRule for crate::message::DiagnosticMessage {
-            fn rule(&self) -> Rule {
-                self.name
-                    .parse()
-                    .unwrap_or_else(|_| unreachable!("invalid rule name: {}", self.name))
-            }
-        }
     }
 }