Rename Diagnostic::syntax_error methods, separate Ord implementation (#19179)

## Summary

This PR addresses some additional feedback on #19053:

- Renaming the `syntax_error` methods to `invalid_syntax` to match the
lint id
- Moving the standalone `diagnostic_from_violation` function to
`Violation::into_diagnostic`
- Removing the `Ord` and `PartialOrd` implementations from `Diagnostic`
in favor of `Diagnostic::start_ordering`

## Test Plan

Existing tests

## Additional Follow-ups

Besides these, I also put the following comments on my todo list, but
they seemed like they might be big enough to have their own PRs:

- [Use `LintId::IOError` for IO
errors](https://github.com/astral-sh/ruff/pull/19053#discussion_r2189425922)
- [Move `Fix` and
`Edit`](https://github.com/astral-sh/ruff/pull/19053#discussion_r2189448647)
- [Avoid so many
unwraps](https://github.com/astral-sh/ruff/pull/19053#discussion_r2189465980)

crates/ruff_linter/src/test.rs

@@ -292,7 +292,7 @@ Either ensure you always emit a fix or change `Violation::FIX_AVAILABILITY` to e
         .chain(parsed.errors().iter().map(|parse_error| {
             create_syntax_error_diagnostic(source_code.clone(), &parse_error.error, parse_error)
         }))
-        .sorted()
+        .sorted_by(Diagnostic::ruff_start_ordering)
         .collect();
     (messages, transformed)
 }
@@ -317,7 +317,7 @@ fn print_syntax_errors(errors: &[ParseError], path: &Path, source: &SourceKind)
 
 /// Print the lint diagnostics in `diagnostics`.
 fn print_diagnostics(mut diagnostics: Vec<Diagnostic>, path: &Path, source: &SourceKind) -> String {
-    diagnostics.retain(|msg| !msg.is_syntax_error());
+    diagnostics.retain(|msg| !msg.is_invalid_syntax());
 
     if let Some(notebook) = source.as_ipy_notebook() {
         print_jupyter_messages(&diagnostics, path, notebook)