ciphers/a1z26: reject non-lowercase input in encode()

[thejesh23]

Describe your change:

Fixes #14931

ciphers.a1z26.encode currently has no input validation and silently produces meaningless output (negative numbers, out-of-range integers) for any character outside a–z:

```

encode("A")
[-31]
encode("HELLO")
[-24, -27, -20, -20, -17]
encode("hi there")
[8, 9, -64, 20, 8, 5, 18, 5]
```

The A1Z26 cipher only defines a–z → 1–26, and the contribution guide asks that erroneous input raise a Python exception rather than silently return wrong values. This PR adds a single-line guard that raises ValueError for the empty string or any character outside a–z, and adds two doctests demonstrating the new behaviour (uppercase input and input containing whitespace).

Lowercase behaviour is unchanged; the existing encode("myname") doctest still passes.

• 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?

(The doctest additions are the direct demonstration of the new validation and cannot be split out meaningfully without leaving the new behaviour untested.)

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".