Add DeletePlaylist support for .NET SDK 3.1.0

[wenbinf]

Adds DeletePlaylist for DELETE /playlists/{id} and prepares PodcastAPI 3.1.0. Applications can permanently delete a playlist using the existing parameter-dictionary convention, Task<ApiResponse>, and optional cancellation token.

The generated README reference explains that deletion is irreversible and that applications should ask for confirmation first. Generated methods, contract, dispatch, and compiled examples now cover 31 operations. Package release notes and handwritten documentation reflect the new method. The .NET 8 minimum and dependencies are unchanged.

Validation:

• Locked restore and Release builds with warnings treated as errors pass under .NET SDK 8.0.425 and 10.0.401.
• All 55 offline tests pass on each runtime, including encoded identifiers, no deletion query/body, unchanged caller parameters, error bodies and headers, cancellation, timeouts, connection failures, and rejected redirects without retries.
• All 31 methods pass against the public mock without credentials or production requests.
• Fresh consumers of the packed NuGet artifact compile every generated README example and verify deletion and empty notes through an in-memory transport. The .NET 10 checks also pass from a source copy outside the monorepo.
• Monorepo generation is deterministic; 52 synchronization tests, 17 Django documentation tests, 84 frontend snippet/runner tests, and frontend typechecking pass.

NuGet publication is separate. The monorepo's published .NET version remains 3.0.0 until the new package is released and verified.