Added multiline cell support for %llm magic in Jupyter

[sankalpG007]

Summary

Enhanced the %llm magic command to support both single-line and multiline inputs in Jupyter Notebooks.

Details

• Implemented cell=None parameter for compatibility with both line and cell magic usage.
• Automatically sets next cell input with generated code when not using --print.
• Ensured compatibility with get_ipython().set_next_input() for seamless workflow.

Why this change

This improves usability by allowing users to write longer prompts or code instructions in cell mode, not just a single line.

Testing

• Verified %llm works as both line and cell magic.
• Code passes black and ruff formatting checks.

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🟩 Hacktoberfest Contribution
This PR adds new functionality following the contribution guidelines.