Optimize Node.has_loop to use constant auxiliary space

[iamshresthraj]

Describe your change

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
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• Documentation change?

Optimize Node.has_loop in data_structures/linked_list/has_loop.py to use Floyd's tortoise-and-hare cycle-finding algorithm:

• Uses two pointers (slow and fast) to detect cycles.
• Compares node references (is) rather than values.
• Reduces auxiliary space complexity from (n)$ to (1)$ while maintaining (n)$ time complexity.
• Accurately detects acyclic lists, self-loops, loops back to earlier nodes/head, loops involving the final node, and lists with duplicate node values.

Fixes #15576

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