AI Gateway 2.1: Support for MCP 2026-07-28 protocol

## Overview

This is the core feature for the 2.1 release: once it's fully developed and tested 2.1 will be ready for GA.

AI Gateway 2.1 adds support for the MCP 2026-07-28 protocol revision, while continuing to support `2025-06-18` and `2025-11-25` on the same MCP endpoint. Protocol revision is resolved per request, with legacy and modern protocol behavior handled independently.

Update the MCP documentation to explain the supported protocol versions, version negotiation, and the client-visible differences introduced by the 2026-07-28 revision.

## Scope

Document (TBD):

* Supported MCP protocol revisions:

* `2025-06-18`
* `2025-11-25`
* `2026-07-28`
* How AI Gateway determines the protocol revision for a request.
* Behavior when an unsupported revision is requested.
* Backward compatibility with legacy MCP clients.
* Differences between legacy and 2026-07-28 clients.

We might want to include a concise legacy vs 2026-07-28 comparison so users can understand which behavior applies to their MCP client.

NOTE: Since the new version of the protocol deprecates the initialise step, we'll most probably need to update our [get started with MCP](https://release-ai-gateway-2-0--kongdeveloper.netlify.app/ai-gateway/get-started-with-mcp-server/)

Size: M to L
Person of contact: Roberto Montalti

PRD: https://docs.google.com/document/d/1rfewif9DfLhAfhcamyn47eLjjc78elDsY-pw2BT3hSI/edit?tab=t.0#heading=h.vxz195lgcpg7

Start with the linked “get started with MCP” page and the PRD, then inventory the existing MCP documentation that covers protocol setup and client behavior. Document the three supported revisions, request negotiation, unsupported revisions, backward compatibility, and the 2026-07-28 differences; update the getting-started page if the initialise-step change requires it.

Written by the indexing model from the issue text.