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/src/main.rs

@@ -14,6 +14,7 @@ use rayon::ThreadPoolBuilder;
 use ruff_db::diagnostic::{Diagnostic, DisplayDiagnosticConfig, Severity};
 use ruff_db::max_parallelism;
 use ruff_db::system::{OsSystem, SystemPath, SystemPathBuf};
+use ruff_db::Upcast;
 use salsa::plumbing::ZalsaDatabase;
 use ty_project::metadata::options::Options;
 use ty_project::watch::ProjectWatcher;
@@ -298,7 +299,11 @@ impl MainLoop {
                             let diagnostics_count = result.len();
 
                             for diagnostic in result {
-                                write!(stdout, "{}", diagnostic.display(db, &display_config))?;
+                                write!(
+                                    stdout,
+                                    "{}",
+                                    diagnostic.display(&db.upcast(), &display_config)
+                                )?;
 
                                 max_severity = max_severity.max(diagnostic.severity());
                             }