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_db/src/parsed.rs

@@ -102,7 +102,7 @@ mod tests {
 
         let parsed = parsed_module(&db, file);
 
-        assert!(parsed.is_valid());
+        assert!(parsed.has_valid_syntax());
 
         Ok(())
     }
@@ -118,7 +118,7 @@ mod tests {
 
         let parsed = parsed_module(&db, file);
 
-        assert!(parsed.is_valid());
+        assert!(parsed.has_valid_syntax());
 
         Ok(())
     }
@@ -134,7 +134,7 @@ mod tests {
 
         let parsed = parsed_module(&db, virtual_file.file());
 
-        assert!(parsed.is_valid());
+        assert!(parsed.has_valid_syntax());
 
         Ok(())
     }
@@ -150,7 +150,7 @@ mod tests {
 
         let parsed = parsed_module(&db, virtual_file.file());
 
-        assert!(parsed.is_valid());
+        assert!(parsed.has_valid_syntax());
 
         Ok(())
     }
@@ -181,6 +181,6 @@ else:
 
         let parsed = parsed_module(&db, file);
 
-        assert!(parsed.is_valid());
+        assert!(parsed.has_valid_syntax());
     }
 }