fix: Updated learn\generation\langchain\handbook\04-langchain-chat.ipynb to Work with LangChain 0.3

[Siraj-Aizlewood]

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
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• None of the above: (explain here)

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[Siraj-Aizlewood]

As well as updating the notebook to use more modern LangChain techniques (e.g. LCEL chains):

1. The old notebook relied on older gpt versions that would output flawed results (which could then be fixed with prompt templates). When replaced with gpt-4.1-mini, these flaws went away, so used new ways of demonstrating uses of prompt templates.
2. Tried to give a concrete example of where LangChain prompt templates might be preferred over a simple dynamically generated f-string in a function. The example involves jinja templates and can be found at the end of the notebook.