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_test/src/lib.rs

@@ -13,6 +13,7 @@ use ruff_db::panic::catch_unwind;
 use ruff_db::parsed::parsed_module;
 use ruff_db::system::{DbWithWritableSystem as _, SystemPath, SystemPathBuf};
 use ruff_db::testing::{setup_logging, setup_logging_with_filter};
+use ruff_db::Upcast;
 use ruff_source_file::{LineIndex, OneIndexed};
 use std::backtrace::BacktraceStatus;
 use std::fmt::Write;
@@ -464,7 +465,7 @@ fn create_diagnostic_snapshot(
             writeln!(snapshot).unwrap();
         }
         writeln!(snapshot, "```").unwrap();
-        write!(snapshot, "{}", diag.display(db, &display_config)).unwrap();
+        write!(snapshot, "{}", diag.display(&db.upcast(), &display_config)).unwrap();
         writeln!(snapshot, "```").unwrap();
     }
     snapshot