feat(plugin-mcp): add support for server instructions (#15858)

### 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`

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

---------

Co-authored-by: Paul Popus <paul@payloadcms.com>

Author: jhb-dev

packages/plugin-mcp/src/mcp/getMcpHandler.ts

@@ -527,6 +527,7 @@ export const getMCPHandler = (
         }
       },
       {
+        instructions: serverOptions.instructions,
         serverInfo: serverOptions.serverInfo,
       },
       {

packages/plugin-mcp/src/types.ts

@@ -373,6 +373,10 @@ export type MCPHandlerOptions = {
  * MCP Server options.
  */
 export type MCPServerOptions = {
+  /**
+   * Optional instructions describing how to use the server and its features.
+   */
+  instructions?: string
   /**
    * Set the server info of the MCP server.
    */