Preserve SyntaxError source line in message on Python 3.12+

Python 3.12 eagerly normalizes the error indicator, so PyErr_Fetch now
hands us the SyntaxError instance (whose str() omits the offending source
line) instead of the raw args tuple (whose str() included it). Callers that
surface PythonException.Message for compile diagnostics therefore lost the
offending source text on 3.12+.

GetMessage now re-appends the SyntaxError 'text' attribute when present.
This is a no-op on <=3.11 (there the fetched value is a tuple without the
SyntaxError attributes) and only affects SyntaxError messages.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Martin-Molinero

src/runtime/PythonException.cs

@@ -252,12 +252,61 @@ private static string GetMessage(PyObject? value, PyType type)
 
             if (value != null && !value.IsNone())
             {
-                return value.ToString() ?? "no message";
+                var message = value.ToString() ?? "no message";
+
+                // Python 3.12+ eagerly normalizes the error indicator, so a SyntaxError
+                // reaches us as the exception instance whose str() omits the offending
+                // source line. Pre-3.12 we received the raw args tuple, whose str()
+                // included it. Re-append the source text so the message stays complete
+                // for callers that surface it (e.g. compile diagnostics). This is a
+                // no-op on <=3.11 (there 'value' is a tuple without these attributes).
+                if (TryGetSyntaxErrorText(value, out var sourceText))
+                {
+                    message = $"{message}: {sourceText}";
+                }
+
+                return message;
             }
 
             return type.Name;
         }
 
+        /// <summary>
+        /// If <paramref name="value"/> is a SyntaxError instance carrying the offending
+        /// source line (its <c>text</c> attribute), returns that trimmed text.
+        /// </summary>
+        private static bool TryGetSyntaxErrorText(PyObject value, out string text)
+        {
+            text = string.Empty;
+            try
+            {
+                // 'msg' + 'text' is the distinctive SyntaxError shape; bail otherwise.
+                if (!value.HasAttr("msg") || !value.HasAttr("text"))
+                {
+                    return false;
+                }
+
+                using var textObj = value.GetAttr("text");
+                if (textObj.IsNone())
+                {
+                    return false;
+                }
+
+                var sourceLine = textObj.ToString();
+                if (string.IsNullOrWhiteSpace(sourceLine))
+                {
+                    return false;
+                }
+
+                text = sourceLine.Trim();
+                return true;
+            }
+            catch (PythonException)
+            {
+                return false;
+            }
+        }
+
         private static string TracebackToString(PyObject traceback)
         {
             if (traceback is null)