Fix traceback circular reference detection to pass test_raise.py

[Copilot]

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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[youknowone]

This is too ad-hoc. You must create a more general logic to detect circular reference in tb_next until the end.

[Copilot]

Implemented a more general cycle detection algorithm using Floyd's cycle detection (tortoise and hare). The new approach simulates the assignment and detects any cycles in the resulting chain rather than only checking for specific patterns. See commit ce5fe45.