Optimize Node.has_loop using Floyd's cycle detection

[Aarya1777]

Summary

• Replace the auxiliary-space approach in Node.has_loop with Floyd's tortoise-and-hare cycle detection algorithm.
• Compare node references using is to detect a cycle.
• Achieve O(n) time complexity and O(1) auxiliary space.

Testing

• The existing examples in the docstring cover lists with and without a loop.
• Tests have not yet been run locally.

Fixes #15576

[algorithms-keeper]

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