Generate a typed client for the Box MCP Server connector

## Description

The SDK registry contains the `boxmcpserver` connector, but SDK 0.14 does not ship a generated typed client for it.

Current state:

- `ConnectorNames.BoxMCPServer` maps to `boxmcpserver`.
- `ManagedConnectors` contains `boxmcpserver` and its generated usage comment references `BoxmcpserverClient`.
- No `BoxmcpserverExtensions.cs`, `BoxmcpserverClient`, or matching DI registration exists in the package.

This prevents a working Connector Namespace `boxmcpserver` connection from being consumed through the typed .NET SDK.

## Expected

Generate the connector client and models from the managed connector contract, add the corresponding DI registration extension, and include constructor, operation, serialization, and error-response tests.

## Context

Found while expanding [Azure/Connectors-NET-Samples#72](https://github.com/Azure/Connectors-NET-Samples/pull/72) to cover authenticated Connector Namespace clients end to end.

Authored with [Dobby](https://aka.ms/dobby).

Start with the managed connector contract and existing generated connector clients, using the missing BoxmcpserverClient and BoxmcpserverExtensions.cs as the target artifacts. Verify that the generated models, matching DI registration, and constructor, operation, serialization, and error-response tests are included and that the boxmcpserver Connector Namespace can be consumed through the typed SDK.

Written by the indexing model from the issue text.