Add docstring to invoke_chain_with_fallback function

[Lang-Qiu]

Problem

The public function invoke_chain_with_fallback in docs/code-comparisons/langchain_snippets/vanilla_fallbacks.py lacked a docstring, which reduced code readability and made it harder for developers to understand its purpose and usage.

Changes

• Added a comprehensive docstring to the function, following Google Python style guidelines, to document its behavior, arguments, and return value.
• The docstring explains that the function attempts to invoke a chain using invoke_chain and falls back to invoke_anthropic_chain on exceptions.
• No functional changes were made; the function's external behavior and API remain unchanged.

Verification

• Code review to ensure the docstring accurately describes the function's logic.
• Confirmed that existing tests pass without modification, as this is a documentation-only update.

[jernejfrank]

Looks good, thanks!