Support starred and dict unpacking in local Python executor

[DivyaNarahari97]

The local Python interpreter rejected several common assignment and dict forms that an LLM writes routinely, so valid agent code failed at runtime.

• Starred assignment targets (a, *b = ...) raised "Cannot unpack tuple of wrong size": set_value compared target and value lengths without accounting for ast.Starred. This affected for-loop targets and comprehensions too, since both assign through set_value.
• List assignment targets ([a, b] = [1, 2]) fell through the target if-chain and silently assigned nothing, leaving the names undefined.
• ** unpacking inside a dict literal ({**d, "b": 2}) raised "NoneType is not supported": for these entries the key node is None, which was passed straight to evaluate_ast. Note **kwargs unpacking in calls was already supported, so the two behaved inconsistently.

set_value now handles ast.Tuple and ast.List targets together, with at most one starred element absorbing the values the fixed targets don't take, and the ast.Dict branch expands ** entries. Genuine size mismatches still raise as before.

Also drops the unreachable ast.Starred branch in evaluate_assign: multiple assign.targets means a chained assignment (a = b = value), which never yields a bare Starred target.

Fixes #2552
Fixes #2555