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_server/src/server/api/requests/diagnostic.rs

@@ -76,8 +76,9 @@ fn to_lsp_diagnostic(
     encoding: crate::PositionEncoding,
 ) -> Diagnostic {
     let range = if let Some(span) = diagnostic.primary_span() {
-        let index = line_index(db.upcast(), span.file());
-        let source = source_text(db.upcast(), span.file());
+        let file = span.expect_ty_file();
+        let index = line_index(db.upcast(), file);
+        let source = source_text(db.upcast(), file);
 
         span.range()
             .map(|range| range.to_lsp_range(&source, &index, encoding))