Implement modified vigenere cipher for all printable ASCII

[Abheelash-Mishra]

Describe your change:

Added a modifed Vigenere cipher script that encrypts and decrypts messages using only printable ASCII characters (32–126). This maintains the original Vigenere cipher behavior with cyclic key repetition, ensures all output remains readable.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

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• I have read CONTRIBUTING.md.
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• Formatted string literals (f-strings)
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[cclauss]

@priya-sundaram-dev, please review.
How is this better, faster, or more correct than our existing ciphers/vigenere_cipher.py?

[priya-sundaram-dev]

Thanks for the ping, @cclauss. I read through it — my honest take:

On "better/faster/more correct" vs ciphers/vigenere_cipher.py: it isn't any of those for the classic cipher. It's the same O(n) translate loop, so no speed win, and for letters-only input the existing file is already correct. What this PR actually is, is a different variant: it operates over the full printable-ASCII range (32–126) instead of A–Z, so it also enciphers digits, punctuation and spaces, and the key is any printable string rather than an alphabetic one. That's a legitimately larger keyspace/message-space, but it's a new cipher, not an improvement to the old one — and the repo generally discourages near-duplicates unless the added value is clear. I'd ask @Abheelash-Mishra to make that case explicitly in the PR description (why the printable-ASCII variant is worth carrying alongside the classic one) so it's a deliberate keep rather than an accidental fork.

If it does stay, two concrete fixes:

1. Empty key crashes instead of erroring cleanly:

   >>> encrypt_message('', 'hi')
   IndexError: list index out of range

   Guard it up front (if not key: raise ValueError("Key must be non-empty")) and add a doctest for the raise.

2. mode is validated inside the per-character loop, so the ValueError only fires if the message has a printable char, and re-checks every iteration. Validate mode once before the loop.

Minor: the module-level PRINTABLE_* constants are nice; consider deriving key_bytes outside translate_message is already good. Doctests pass locally (6/6). Overall the code is clean and readable — the real question is scope/duplication, which is your call as maintainer.