Extend REST client for LineServing endpoints

[ans-ioki]

Context

This is a child of the existing dbdrive/[backlog#3164](https://github.com/dbdrive/backlog/issues/3164) ("🔀🚏
TaskLists with multiple LineServings") and dbdrive/[triebwerk#20900](https://github.com/dbdrive/triebwerk/issues/20900)
("Line Servings").

Note: this repo lives in a different GitHub org (ioki-mobility).

Scope

• Extend this gem's REST client for the new LineServing endpoints
  introduced by dbdrive/[triebwerk#20900](https://github.com/dbdrive/triebwerk/issues/20900) (CRUD routes for LineServing
  as a TaskList's new supply-side association, replacing the old
  single bound Line).

Acceptance Criteria

• The gem exposes create/read/update/delete methods for LineServing, following the same resource-client pattern already used for other Platform API resources in this gem.
• Each method is covered by a test against a stubbed/recorded API response.
• The gem's version is bumped and its changelog updated per this repo's release process.
• Existing gem consumers (e.g. ioki-cli, sim-ioki) are unaffected by this addition — no breaking change to existing resource clients.