Add delete_playlist support for Rust SDK 3.1.0

[wenbinf]

Adds Client::delete_playlist(id, &json!({})) for DELETE /playlists/{id} and prepares podcast-api 3.1.0. The generated README includes the permanent-deletion warning and a runnable public-mock example; the method preserves the existing Response wrapper and positional path-argument convention.

The contract and test dispatcher now cover 31 methods. Focused tests verify encoded playlist IDs, an empty DELETE body, response headers, API errors, client isolation, validation, timeouts, and disabled redirects/retries. Rust 1.88 remains the minimum; dependencies are unchanged.

Validation:

• Rust 1.88.0 and 1.98.1: 8 offline tests and 33 compiled documentation examples passed on each.
• All 3 public-mock integration tests passed, including all 31 methods; no production API requests.
• Formatting, Clippy, documentation build, and standalone crate packaging/extracted-package tests passed.
• Monorepo: 52 synchronization tests, checks for all 11 SDKs, 17 Django documentation tests, frontend typechecking, and 84 snippet/runner tests passed.

Crates.io publication is separate. The website's published-version gate remains at 3.0.0 until the new crate is available.