Add dependency injection registration for CloudConvertClient

## Description

SDK 0.14 adds the generated `CloudconvertClient` and `ConnectorNames.CloudConvert`, but `ConnectorServiceCollectionExtensions` does not provide an `AddCloudconvertClient` method.

The samples use SDK DI registration extensions so clients receive their configured connection runtime URL and managed-identity credential. CloudConvert cannot follow that supported pattern even though its typed client is present.

## Expected

Add `AddCloudconvertClient(IServiceCollection, IConfiguration)` using `ConnectorNames.CloudConvert`, consistent with the other connector registrations, and add registration tests.

A separate validation connection is still needed for deployed E2E coverage, but the missing DI extension is an SDK package gap.

## Context

Found while expanding [Azure/Connectors-NET-Samples#72](https://github.com/Azure/Connectors-NET-Samples/pull/72) for SDK 0.14.

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

Open ConnectorServiceCollectionExtensions and compare the existing connector registration methods; then locate CloudconvertClient and ConnectorNames.CloudConvert. Add the matching registration tests and verify the client receives the configured runtime URL and managed-identity credential consistently with the other connectors.

Written by the indexing model from the issue text.