Add test implementation of an "mcp" command

[jedisct1]

This introduces a new mcp command that allows the Fastly CLI tool to act as an MCP (Model Context Protocol) server.

Currently, it serves the Fastly API, but the design is intended to support additional MCP servers in the future, for example, to assist with Compute SDKs.

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All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

• Does your submission pass tests?

Changes to Core Features:

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• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

User Impact

• What is the user impact of this change?

User Impact:
This change enables users to run the Fastly CLI as an MCP server, allowing integration with tools and editors that support the MCP protocol. Initially, this provides access to the Fastly API via MCP, and the architecture is designed to support additional MCP server types in the future (e.g., for Compute SDKs). This opens up new automation and integration possibilities for users and partners.

Are there any considerations that need to be addressed for release?

No breaking changes are introduced. The new mcp command is additive and does not affect existing CLI functionality.
Note: When running in stdio mode, all server startup messages are now sent to stderr to avoid interfering with the MCP protocol.