Fixes the traceback tb_next setter to properly detect and prevent circular references, enabling test_raise.py::TestTracebackType.test_attrs to pass.

Problem

The test TestTracebackType.test_attrs in test_raise.py was marked with @unittest.expectedFailure because the traceback implementation was missing circular reference detection. The test expects:

# These should raise ValueError due to circular references
tb.tb_next = tb  # Direct self-assignment 
tb.tb_next.tb_next = tb  # Indirect loop through chain

But the current implementation allowed these assignments without validation.

Solution

Added circular reference detection to the set_tb_next method in vm/src/builtins/traceback.rs:

1. Direct loop detection: Checks if the new traceback being assigned is the same object as self
2. Indirect loop detection: Walks the traceback chain from the new object to ensure self doesn't appear anywhere in the chain
3. Proper pointer comparison: Uses std::ptr::eq with correct dereferencing to compare the underlying PyTraceback objects

The implementation raises ValueError("circular reference in traceback chain") when a circular reference is detected, matching CPython's behavior.

Changes

• Removed @unittest.expectedFailure decorator from TestTracebackType.test_attrs
• Added loop detection logic to PyTraceback::set_tb_next()
• Added std::ops::Deref import for proper object dereferencing

Testing

• ✅ TestTracebackType.test_attrs now passes
• ✅ All 36 tests in test_raise.py continue to pass
• ✅ No clippy warnings introduced

The fix is minimal and surgical, only adding the necessary validation without changing any existing working functionality.

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