Update caesar_cipher.py

[UdayBadola]

Input Validation and Error Handling:

Implemented the get_valid_integer function to ensure that the key entered by the user is a valid integer.
Added prompts for the user to input a custom alphabet or use the default one.
Refactoring Code Structure:

Encapsulated the main menu logic within a main function.
Separated concerns by creating helper functions for input validation.
PEP 8 Compliance:

Adjusted code formatting, such as adding spaces around operators and after commas.
Ensured that line lengths do not exceed 79 characters where possible.
Enhanced Docstrings:

Updated docstrings to provide clearer and more concise explanations.
Used consistent formatting for parameters and return values.
Type Hinting:

Used Optional[str] from the typing module for better type hinting.
Specified return types for all functions.
Modularization:

Separated user interaction from the core logic, making the code more modular and easier to test.
This separation allows the core functions to be reused in other modules or scripts.
User Experience Improvements:

Enhanced the main menu to display options in a more readable format.
Allowed users to specify a custom alphabet, providing more flexibility.
Added clear prompts to guide the user through the steps.
Edge Case Handling:

Improved handling of large key values by using modulo operation to wrap around the alphabet.
Ensured that the cipher works correctly even when the key is larger than the length of the alphabet.

[algorithms-keeper]

Closing this pull request as invalid

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