feat(plugin-mcp): add support for server instructions

[jhb-dev]

What?

Adds an instructions option to the MCP plugin's MCPServerOptions type.

Why?

The MCP protocol supports an instructions field that is sent to clients during initialization. The Payload MCP plugin currently doesn't expose this option, so users have no way to provide instructions describing how to use the server and its features.

How?

• Added instructions field to MCPServerOptions in types.ts
• Pass instructions through to the MCP SDK via createMcpHandler in getMcpHandler.ts

mcpPlugin({
  collections: { ... },
  mcp: {
    serverOptions: {
      instructions: 'This server manages blog posts and pages.',
    },
  },
})

[kendelljoseph]

Cool! Makes sense to me!