fix(ciphers): add input validation and doctests to a1z26 encode and decode

[amir-rezaei]

Description

This PR resolves issue #14931 by adding input validation to encode() and decode() in ciphers/a1z26.py.

Details

• Raises ValueError when encode() receives non-lowercase inputs or decode() receives integers out of the 1–26 alphabet index range.
• Includes doctests verifying exception behavior.

[algorithms-keeper]

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