Add DeletePlaylist support for Go SDK 3.1.0

[wenbinf]

Adds DeletePlaylist(id, nil) for DELETE /playlists/{id} and prepares the Go SDK's v3.1.0 release. Generated methods, the contract, compiled examples, and README now cover 31 operations, including the permanent-deletion warning and a public-mock example.

Existing method signatures and response/error handling stay unchanged. The exported HTTPClient interface gains DeletePlaylist, so custom implementations and mocks must add that method; this compatibility requirement is documented in the README. Go 1.26 and the /v3 import path remain unchanged, with no third-party dependencies.

Validation:

• Race tests, compiled examples, vet, build, formatting, and module-tidiness checks on Go 1.26.8 and 1.27.1.
• Public-mock integration across all 31 operations, including playlist deletion; no production API calls.
• Independent source-copy tests and a fresh consumer using a local module replacement and loopback HTTP fixture.
• Monorepo synchronization tests and checks for all 11 SDKs, Django documentation tests, frontend typechecking, and snippet/runner tests.

Publishing the version tag remains separate. The website registry keeps published_version at 3.0.0 until v3.1.0 is published and verified.