fix: validate Butterworth filter inputs - #14979

Describe your change:

Fixes #12379

Adds shared validation for Butterworth filter factory inputs before coefficients are calculated. The change rejects non-positive frequencies, samplerates, and Q factors with ValueError, and rejects cutoff frequencies at or above the Nyquist limit. This prevents invalid high-pass/low-pass/etc. calls from returning unusable coefficients or falling through to ZeroDivisionError. The new doctests cover valid input and the expected error cases.

Validation:

• python3 -m doctest -v audio_filters/butterworth_filter.py - passed, 19 doctests passed.
• python3 -m pytest audio_filters/butterworth_filter.py - passed, 8 tests passed.
• uvx ruff check audio_filters/butterworth_filter.py - passed.
• python3 -m compileall -q audio_filters/butterworth_filter.py - passed.
• git diff --check - passed.

Blockers:

• uv run --with=pytest pytest audio_filters/butterworth_filter.py could not start locally because no Python >=3.14 interpreter is available on this machine, and uv python install 3.14 reported no macOS/aarch64 download for that request.
• python3 -m pytest audio_filters collected the touched file successfully, but the broader package run is blocked locally by missing optional dependencies in unrelated files: audio_filters/show_response.py requires matplotlib, and after ignoring it, audio_filters/iir_filter.py requires scipy for its doctest.
• uvx ruff check . with local ruff 0.16.0 reports three unrelated existing findings outside this PR's file; the touched file passes the same ruff command.

• 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. 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".