Support starred and iterable unpacking in assignments

[himanshu748]

Fixes #2555.

What happens today

Several standard unpacking forms that models write constantly are rejected by the local executor, with errors that point nowhere:

a, *b = [1, 2, 3]        # InterpreterError: Cannot unpack tuple of wrong size
first, *rest = "hello"   # InterpreterError: Cannot unpack tuple of wrong size
a, b = "hi"              # InterpreterError: Cannot unpack non-tuple value
[a, b] = [1, 2]          # silently assigns nothing

set_value only handled ast.Tuple targets: it required an exact length match (so any starred target failed the size check), it excluded str/bytes from the iterable path (so string unpacking was refused), and it had no ast.List branch at all (so [a, b] = ... was silently dropped). The model gets no signal that * targets simply are not implemented, so it retries the same valid code.

The fix

Rewrite the tuple/list target branch to match CPython:

• accept any iterable, not just tuples and lists
• support a single starred target that absorbs the surplus into a list, in any position (a, *b, *a, b, a, *b, c)
• handle ast.List targets alongside ast.Tuple
• raise CPython-parity messages: not enough values to unpack (expected N, got M), too many values to unpack (expected N), not enough values to unpack (expected at least N, got M), cannot unpack non-iterable X object, and multiple starred expressions in assignment

Nested targets (a, (b, c) = 1, (2, 3)) keep working through the existing recursion.

Tests

14 new parametrized cases covering starred unpacking in every position, empty-star, string/list-pattern/nested unpacking, and the four error messages. One existing test (test_evaluate_assign_error) was updated from the old "Cannot unpack tuple of wrong size" wording to the new CPython message. All new cases fail on current main. Full test file: 411 passed, 2 skipped. Ruff check and format are clean.

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