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/ruff_server/src/lint.rs

@@ -14,7 +14,7 @@ use ruff_linter::{
     directives::{extract_directives, Flags},
     generate_noqa_edits,
     linter::check_path,
-    message::{DiagnosticMessage, Message, SyntaxErrorMessage},
+    message::{DiagnosticMessage, Message},
     package::PackageRoot,
     packaging::detect_package_root,
     registry::AsRule,
@@ -173,10 +173,10 @@ pub(crate) fn check(
                     locator.to_index(),
                     encoding,
                 )),
-                Message::SyntaxError(syntax_error_message) => {
+                Message::SyntaxError(_) => {
                     if show_syntax_errors {
                         Some(syntax_error_to_lsp_diagnostic(
-                            syntax_error_message,
+                            &message,
                             &source_kind,
                             locator.to_index(),
                             encoding,
@@ -322,7 +322,7 @@ fn to_lsp_diagnostic(
 }
 
 fn syntax_error_to_lsp_diagnostic(
-    syntax_error: SyntaxErrorMessage,
+    syntax_error: &Message,
     source_kind: &SourceKind,
     index: &LineIndex,
     encoding: PositionEncoding,
@@ -331,7 +331,7 @@ fn syntax_error_to_lsp_diagnostic(
     let cell: usize;
 
     if let Some(notebook_index) = source_kind.as_ipy_notebook().map(Notebook::index) {
-        NotebookRange { cell, range } = syntax_error.range.to_notebook_range(
+        NotebookRange { cell, range } = syntax_error.range().to_notebook_range(
             source_kind.source_code(),
             index,
             notebook_index,
@@ -340,7 +340,7 @@ fn syntax_error_to_lsp_diagnostic(
     } else {
         cell = usize::default();
         range = syntax_error
-            .range
+            .range()
             .to_range(source_kind.source_code(), index, encoding);
     }
 
@@ -353,7 +353,7 @@ fn syntax_error_to_lsp_diagnostic(
             code: None,
             code_description: None,
             source: Some(DIAGNOSTIC_NAME.into()),
-            message: syntax_error.message,
+            message: syntax_error.body().to_string(),
             related_information: None,
             data: None,
         },