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_python_semantic/src/types/context.rs

@@ -521,7 +521,7 @@ impl Drop for DiagnosticGuard<'_, '_> {
         };
 
         let expected_file = self.ctx.file();
-        let got_file = ann.get_span().file();
+        let got_file = ann.get_span().expect_ty_file();
         assert_eq!(
             expected_file,
             got_file,