Convert Message::SyntaxError to use Diagnostic internally (#17784)

## Summary

This PR is a first step toward integration of the new `Diagnostic` type
into ruff. There are two main changes:
- A new `UnifiedFile` enum wrapping `File` for red-knot and a
`SourceFile` for ruff
- ruff's `Message::SyntaxError` variant is now a `Diagnostic` instead of
a `SyntaxErrorMessage`

The second of these changes was mostly just a proof of concept for the
first, and it went pretty smoothly. Converting `DiagnosticMessage`s will
be most of the work in replacing `Message` entirely.

## Test Plan

Existing tests, which show no changes.

---------

Co-authored-by: Carl Meyer <carl@astral.sh>
Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ty_project/src/db.rs

@@ -126,6 +126,16 @@ impl Upcast<dyn IdeDb> for ProjectDatabase {
     }
 }
 
+impl Upcast<dyn Db> for ProjectDatabase {
+    fn upcast(&self) -> &(dyn Db + 'static) {
+        self
+    }
+
+    fn upcast_mut(&mut self) -> &mut (dyn Db + 'static) {
+        self
+    }
+}
+
 #[salsa::db]
 impl IdeDb for ProjectDatabase {}