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_linter/src/linter.rs

@@ -42,6 +42,8 @@ pub struct LinterResult {
     pub messages: Vec<Message>,
     /// A flag indicating the presence of syntax errors in the source file.
     /// If `true`, at least one syntax error was detected in the source file.
+    ///
+    /// This includes both [`ParseError`]s and [`UnsupportedSyntaxError`]s.
     pub has_syntax_error: bool,
 }
 
@@ -133,7 +135,7 @@ pub fn check_path(
     }
 
     // Run the AST-based rules only if there are no syntax errors.
-    if parsed.is_valid() {
+    if parsed.has_valid_syntax() {
         let use_ast = settings
             .rules
             .iter_enabled()
@@ -283,7 +285,7 @@ pub fn check_path(
             locator,
             comment_ranges,
             &directives.noqa_line_for,
-            parsed.is_valid(),
+            parsed.has_valid_syntax(),
             &per_file_ignores,
             settings,
         );
@@ -294,7 +296,7 @@ pub fn check_path(
         }
     }
 
-    if parsed.is_valid() {
+    if parsed.has_valid_syntax() {
         // Remove fixes for any rules marked as unfixable.
         for diagnostic in &mut diagnostics {
             if !settings.rules.should_fix(diagnostic.kind.rule()) {
@@ -445,7 +447,7 @@ pub fn lint_only(
             &locator,
             &directives,
         ),
-        has_syntax_error: !parsed.is_valid() || !parsed.unsupported_syntax_errors().is_empty(),
+        has_syntax_error: parsed.has_syntax_errors(),
     }
 }
 
@@ -546,7 +548,7 @@ pub fn lint_fix<'a>(
         );
 
         if iterations == 0 {
-            is_valid_syntax = parsed.is_valid() && parsed.unsupported_syntax_errors().is_empty();
+            is_valid_syntax = parsed.has_no_syntax_errors();
         } else {
             // If the source code was parseable on the first pass, but is no
             // longer parseable on a subsequent pass, then we've introduced a