Fix undetected integer overflow when converting to Int64 on 32-bit

On 32-bit, the TypeCode.Int64 path uses PyLong_AsLongLong, whose wrapper
returns a nullable long? that is null when the Python int does not fit in a
long long (with a Python OverflowError left set). The overflow check compared
the nullable to -1 (`num == -1`), which is never true for null, so an
overflowing value bypassed the check and was returned as a successful
conversion with a null result.

Check num.HasValue instead so overflow propagates as a failed conversion.

This is why TestConverter.ConvertOverflow failed only on Windows x86: on x64
the Int64 case takes the else branch (PyLong_AsSignedSize_t, a 64-bit nint)
whose `num == -1 && ErrorOccurred()` check works correctly. The CI matrix only
builds x86 on Windows, so the 32-bit bug surfaced only there.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: jhonabreul

src/runtime/Converter.cs

@@ -1151,11 +1151,17 @@ internal static bool ToPrimitive(BorrowedReference value, Type obType, out objec
                                 goto type_error;
                             }
                             long? num = Runtime.PyLong_AsLongLong(value);
-                            if (num == -1 && Exceptions.ErrorOccurred())
+                            // PyLong_AsLongLong already returns null when the value
+                            // does not fit in a long long (it leaves a Python
+                            // OverflowError set). Comparing the nullable to -1 never
+                            // matched that null, so on 32-bit an overflowing value
+                            // was silently accepted and returned as a null result.
+                            // Check HasValue so the overflow propagates.
+                            if (!num.HasValue)
                             {
                                 goto convert_error;
                             }
-                            result = num;
+                            result = num.Value;
                             return true;
                         }
                         else