fix(python): enforce recursion depth limits in monty_to_py and py_to_monty

[chaliy]

Summary

• Add depth tracking to monty_to_py and py_to_monty conversion functions
• Enforce MAX_NESTING_DEPTH = 64 consistent with existing JSON conversion limits
• Return PyValueError when depth exceeded instead of stack overflow
• Follows exact same pattern as json_to_py_inner / py_to_json_inner

Test plan

• Deeply nested structures (>64 levels) return graceful error
• Normal depth structures convert successfully
• Existing embedded Python builtin tests pass
• All cargo tests pass

Closes #986