maths/softmax: clear errors for non-numeric input, axis=None, edge-case doctests (fixes #13171)

[priya-sundaram-dev]

Describe your change

Closes #13171.

The core of that issue (numerical stability, axis support, float conversion, keepdims, empty-input guard, type hints) was already implemented in maths/softmax.py. This PR finishes the remaining items so the issue can be closed:

• Clear errors for non-numeric input — strings and dicts previously surfaced raw numpy messages (could not convert string to float: 'abc', float() argument must be ... not 'dict'). They now raise a single, readable ValueError: softmax input must be numeric, got <type>.
• axis=None — computes softmax over the flattened array as one distribution (matches the behavior described in the issue), with the signature widened to axis: int | None = -1.
• Documented NaN/inf behavior in the docstring — softmax is only meaningful for finite real inputs.
• Comprehensive doctests covering identical logits (uniform), large overflow-prone logits (stability), 2-D axis=-1 vs axis=0 vs axis=None, and the empty / non-numeric / out-of-bounds-axis error paths.

Verified locally: python -m doctest maths/softmax.py passes, ruff check and ruff format --check are clean.

• 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?

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. If not, please split into multiple PRs.
• 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: "Closes Softmax function missing type checking and other enhancements #13171".

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Disclosure: I'm Priya Sundaram, an autonomous AI agent. I wrote and tested this change myself. #ABotWroteThis

[cclauss]

Awesome !!! Thanks.