SEP-2350: Clarify client-side scope accumulation in step-up authorization

**Is your feature request related to a problem? Please describe.**

Implement support for [SEP-2350], which is planned for inclusion in the 2026-06-30 version of the MCP spec.

[SEP-2350]: https://github.com/modelcontextprotocol/modelcontextprotocol/issues/2350

**Describe the solution you'd like**

Compliance with the 2026-06-30 version of the MCP spec within two weeks of its release.