fix(vm): keep str's concat error when __radd__ declines (#8652)

`PyStr::__add__` delegated to the right operand's `__radd__` whenever it had
one, and returned whatever came back. For every numeric type that is a
`NotImplemented`, which then reached the generic binary-op handler:

    >>> "a" + 1
    TypeError: unsupported operand type(s) for +: 'str' and 'int'

CPython falls back to `str`'s `sq_concat` once the reflected call declines, so
it names the concatenation instead:

    TypeError: can only concatenate str (not "int") to str

The specific message was already here, but only reachable for operands with no
`__radd__` at all, so `list`, `object`, `bytes` and `None` were correct while
`int`, `float`, `bool` and any class with a declining `__radd__` were not.

A `__radd__` that returns a value still wins, unchanged.

Verified against CPython 3.14.0: the new snippet asserts the message for
`int`, `float`, `bool`, `list`, `None`, `bytes` and a declining `__radd__`,
and passes under both interpreters. Reverting only the `str.rs` change fails
its first assertion.

Assisted-by: Claude Code:claude-opus-5

crates/vm/src/builtins/str.rs

@@ -653,10 +653,17 @@ impl PyStr {
                 Self::new_str_unchecked(bytes.into(), kind)
             }
             .to_pyobject(vm))
-        } else if let Some(radd) = vm.get_method(other.clone(), identifier!(vm, __radd__)) {
-            // hack to get around not distinguishing number add from seq concat
-            radd?.call((zelf,), vm)
         } else {
+            // hack to get around not distinguishing number add from seq concat
+            if let Some(radd) = vm.get_method(other.clone(), identifier!(vm, __radd__)) {
+                let result = radd?.call((zelf,), vm)?;
+                // CPython reaches `str`'s sq_concat once the reflected call declines, so a
+                // `__radd__` returning NotImplemented must still report the concat error
+                // rather than the generic binary-op one.
+                if !result.is(&vm.ctx.not_implemented) {
+                    return Ok(result);
+                }
+            }
             Err(vm.new_type_error(format!(
                 r#"can only concatenate str (not "{}") to str"#,
                 other.class().slot_name()

extra_tests/snippets/builtin_str.py

@@ -979,3 +979,39 @@ def test_expandtabs_zero_tabsize():
 
 
 test_expandtabs_zero_tabsize()
+
+
+def test_concat_error_message():
+    # CPython falls back to str's sequence concatenation once a reflected
+    # __radd__ declines, so the message names the concatenation, not the
+    # generic binary operation. Types that define __radd__ (every number)
+    # used to take the generic path here.
+    def message(other):
+        try:
+            "a" + other
+        except TypeError as err:
+            return str(err)
+        raise AssertionError("TypeError was not raised")
+
+    assert message(1) == 'can only concatenate str (not "int") to str'
+    assert message(1.5) == 'can only concatenate str (not "float") to str'
+    assert message(True) == 'can only concatenate str (not "bool") to str'
+    assert message([]) == 'can only concatenate str (not "list") to str'
+    assert message(None) == 'can only concatenate str (not "NoneType") to str'
+    assert message(b"b") == 'can only concatenate str (not "bytes") to str'
+
+    class Declines:
+        def __radd__(self, other):
+            return NotImplemented
+
+    assert message(Declines()) == 'can only concatenate str (not "Declines") to str'
+
+    # A __radd__ that returns a value still wins.
+    class Accepts:
+        def __radd__(self, other):
+            return "from-radd"
+
+    assert "a" + Accepts() == "from-radd"
+
+
+test_concat_error_message()