Introduce MicrosoftGraphTokenProvider to obtain and cache OAuth2 client-credentials tokens from Microsoft Graph. The provider validates required credentials, posts to the tenant token endpoint via Guzzle, throws a RuntimeException on missing creds or HTTP errors (propagating server error messages), and caches the access token for ~50 minutes using a server-specific cache key. Includes unit tests that assert exception behavior for missing credentials and mock Guzzle to verify token request parameters and caching behavior.

Add an optional Guzzle Client dependency to MicrosoftGraphTokenProvider so an HTTP client can be injected for testing. The constructor now accepts ?Client $httpClient and the provider uses $this->httpClient ?? new Client() when posting for tokens. Update the unit test to mock GuzzleHttp\Client (remove overload-based mock and separate-process annotations), import Client, and pass the mocked client into the provider. This enables dependency injection and simpler, more reliable tests.

Replace Mockery-based response mocks in MicrosoftGraphTokenProviderTest with an actual GuzzleHttp\Psr7\Response instance. Simplifies the test by providing a real PSR-7 response (200) with a JSON body and removes the need to mock getBody()/getContents() and getStatusCode(). Added the Response use statement.

Add Microsoft Graph token provider with client credentials flow and unit tests