fix(guardrails): wire _unpack_sequence_ into the custom-code sandbox

RestrictedPython rewrites every tuple-unpacking target that is not a for-loop
target into a call to _unpack_sequence_, and ships no default for it -- the
same way it ships no _inplacevar_, which this module already supplies. The
sandbox globals never defined it, so ordinary guardrail code compiles cleanly
and then raises NameError on its first run:

    a, b = pair
    a, (b, c) = nested
    a, *rest = seq
    with ctx as (a, b):

RestrictedPython.Guards.guarded_unpack_sequence is the helper the rewritten
bytecode expects, and it applies the same _getiter_ guard to each element that
guarded_iter_unpack_sequence already applies for `for a, b in x`.

Rebuilt on `main`: this PR was opened against `litellm_internal_staging`,
which has since been deleted, and `main` diverged from it rather than
containing it. The change is re-applied on top of current `main`, where the
guard is still absent from build_sandbox_globals(); the test moved upstream to
tests/unit/proxy/guardrails/ and the new cases are appended there.

tests/unit/proxy/guardrails/test_custom_code_security.py -> 61 passed. With
the one globals entry removed, the four new cases fail with exactly
`NameError: name '_unpack_sequence_' is not defined`.

litellm/proxy/guardrails/guardrail_hooks/custom_code/sandbox.py

@@ -31,6 +31,7 @@ from RestrictedPython.Eval import default_guarded_getitem
 from RestrictedPython.Guards import (
     full_write_guard,
     guarded_iter_unpack_sequence,
+    guarded_unpack_sequence,
     safer_getattr,
 )
 from RestrictedPython.transformer import copy_locations
@@ -137,6 +138,12 @@ def build_sandbox_globals() -> dict[str, object]:
         "_getitem_": default_guarded_getitem,
         "_getiter_": budgeted_iter,
         "_iter_unpack_sequence_": guarded_iter_unpack_sequence,
+        # RestrictedPython emits _unpack_sequence_ for every tuple-unpacking
+        # target that is not a for-loop target -- ``a, b = pair``,
+        # ``a, *rest = seq``, ``with x as (a, b)`` -- and, like _inplacevar_,
+        # ships no default. Without it those statements compile and then raise
+        # NameError the first time the guardrail runs.
+        "_unpack_sequence_": guarded_unpack_sequence,
         "_write_": full_write_guard,
         "_inplacevar_": _inplacevar_,
         "_budget_ok_": budget_ok,

tests/unit/proxy/guardrails/test_custom_code_security.py

@@ -846,3 +846,32 @@ def test_config_without_timeout_uses_the_default():
         _initialize_from_config("custom-code-default-timeout", {}).execution_timeout
         == DEFAULT_EXECUTION_TIMEOUT_SECONDS
     )
+
+
+@pytest.mark.parametrize(
+    ("body", "expected"),
+    [
+        # Every one of these compiles fine and then raises
+        # "NameError: name '_unpack_sequence_' is not defined" at call time when
+        # the guard is missing from the sandbox globals.
+        ("    a, b = inputs['pair']\n    return a + b\n", 3),
+        ("    a, (b, c) = inputs['nested']\n    return a + b + c\n", 6),
+        ("    a, *rest = inputs['seq']\n    return rest\n", [2, 3]),
+        ("    with inputs['ctx'] as (a, b):\n        return a + b\n", 15),
+    ],
+)
+def test_tuple_unpacking_runs(body: str, expected):
+    """Ordinary tuple unpacking must work inside a guardrail, not NameError."""
+
+    class _Ctx:
+        def __enter__(self):
+            return (7, 8)
+
+        def __exit__(self, *args):
+            return False
+
+    guardrail = _compile("def apply_guardrail(inputs, request_data, input_type):\n" + body)
+    fn = guardrail._compiled_function
+    assert fn is not None
+    inputs = {"pair": (1, 2), "nested": (1, (2, 3)), "seq": [1, 2, 3], "ctx": _Ctx()}
+    assert fn(inputs, {}, "request") == expected