Document LinterResult::has_syntax_error and add Parsed::has_no_syntax_errors (#16443)

Summary
--

This is a follow up addressing the comments on #16425. As @dhruvmanila
pointed out, the naming is a bit tricky. I went with `has_no_errors` to
try to differentiate it from `is_valid`. It actually ends up negated in
most uses, so it would be more convenient to have `has_any_errors` or
`has_errors`, but I thought it would sound too much like the opposite of
`is_valid` in that case. I'm definitely open to suggestions here.

Test Plan
--

Existing tests.

crates/ruff_python_parser/tests/fixtures.rs

@@ -39,9 +39,7 @@ fn test_valid_syntax(input_path: &Path) {
     });
     let parsed = parse_unchecked(&source, options);
 
-    let is_valid = parsed.is_valid() && parsed.unsupported_syntax_errors().is_empty();
-
-    if !is_valid {
+    if parsed.has_syntax_errors() {
         let line_index = LineIndex::from_source_text(&source);
         let source_code = SourceCode::new(&source, &line_index);
 
@@ -101,10 +99,8 @@ fn test_invalid_syntax(input_path: &Path) {
     });
     let parsed = parse_unchecked(&source, options);
 
-    let is_valid = parsed.is_valid() && parsed.unsupported_syntax_errors().is_empty();
-
     assert!(
-        !is_valid,
+        parsed.has_syntax_errors(),
         "{input_path:?}: Expected parser to generate at least one syntax error for a program containing syntax errors."
     );
 
@@ -224,7 +220,7 @@ f'{foo!r'
     println!("AST:\n----\n{:#?}", parsed.syntax());
     println!("Tokens:\n-------\n{:#?}", parsed.tokens());
 
-    if !parsed.is_valid() {
+    if parsed.has_invalid_syntax() {
         println!("Errors:\n-------");
 
         let line_index = LineIndex::from_source_text(source);