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_server/src/lint.rs

@@ -163,7 +163,7 @@ pub(crate) fn check(
             .into_iter()
             .zip(noqa_edits)
             .filter_map(|(message, noqa_edit)| {
-                if message.is_syntax_error() && !show_syntax_errors {
+                if message.is_invalid_syntax() && !show_syntax_errors {
                     None
                 } else {
                     Some(to_lsp_diagnostic(