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/ty_test/src/lib.rs

@@ -322,14 +322,14 @@ fn run_test(
             let mut diagnostics: Vec<Diagnostic> = parsed
                 .errors()
                 .iter()
-                .map(|error| Diagnostic::syntax_error(test_file.file, &error.error, error))
+                .map(|error| Diagnostic::invalid_syntax(test_file.file, &error.error, error))
                 .collect();
 
             diagnostics.extend(
                 parsed
                     .unsupported_syntax_errors()
                     .iter()
-                    .map(|error| Diagnostic::syntax_error(test_file.file, error, error)),
+                    .map(|error| Diagnostic::invalid_syntax(test_file.file, error, error)),
             );
 
             let mdtest_result = attempt_test(db, check_types, test_file, "run mdtest", None);