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_ide/src/lib.rs

@@ -204,6 +204,7 @@ mod tests {
     use ruff_db::diagnostic::{Diagnostic, DiagnosticFormat, DisplayDiagnosticConfig};
     use ruff_db::files::{system_path_to_file, File};
     use ruff_db::system::{DbWithWritableSystem, SystemPath, SystemPathBuf};
+    use ruff_db::Upcast;
     use ruff_python_ast::PythonVersion;
     use ruff_text_size::TextSize;
     use ty_python_semantic::{
@@ -285,7 +286,7 @@ mod tests {
                 .format(DiagnosticFormat::Full);
             for diagnostic in diagnostics {
                 let diag = diagnostic.into_diagnostic();
-                write!(buf, "{}", diag.display(&self.db, &config)).unwrap();
+                write!(buf, "{}", diag.display(&self.db.upcast(), &config)).unwrap();
             }
 
             buf