Add delete_playlist support for Python SDK 3.1.0

[wenbinf]

Adds client.delete_playlist(id=...) for DELETE /playlists/{id} and prepares Python SDK 3.1.0. It keeps the existing keyword-argument interface and requests.Response return value. The generated README explains permanent deletion and the developer's responsibility to add a confirmation step in their application's UI.

Regenerates the method and API contract; extends coverage for encoded playlist identifiers, missing identifiers, bodyless DELETE requests, and preventing retries of failed write responses. Adds a public-mock deletion test and removes the old hard-coded version from the User-Agent assertion.

Validation:

• 84 offline tests passed on each of Python 3.10 and 3.14; 99% coverage on 3.14.
• 13 integration tests passed against the credential-free public mock API.
• Black, Flake8, deterministic generation, and sync.py python --check passed.
• Wheel and source distribution built and passed uv publish --dry-run; the wheel was installed and verified outside the checkout with mocked HTTP.

PyPI publication is separate. The monorepo keeps Python documentation availability at 3.0.0 until 3.1.0 is published.