Add IncludeResourceIndicator option for OAuth and DI client sample

[Varun6578]

Summary

This PR addresses two issues from the Backlog milestone:

Fix #648: OAuth authentication fails with MS Entra-ID

The SDK sends a resource parameter (RFC 8707) in OAuth authorization, token exchange, and token refresh requests. MS Entra-ID v2.0 does not support this parameter and returns AADSTS901002.

Changes:

• Added IncludeResourceIndicator property to ClientOAuthOptions (defaults to true for backward compatibility)
• When set to false, the resource parameter is omitted from all OAuth requests
• Added test verifying the authorization URL omits the resource parameter when the option is disabled

Usage:

OAuth = new()
{
    ClientId = "my-client",
    RedirectUri = new Uri("http://localhost:1179/callback"),
    IncludeResourceIndicator = false, // For Entra-ID compatibility
    AuthorizationRedirectDelegate = HandleAuthorizationUrlAsync,
}

Fix #147: Add client sample with DI guidance

• Created samples/DependencyInjectionClient showing how to register McpClient as a singleton via IServiceCollection and consume it from an IHostedService
• Added DI wiring + Docker transport guidance section to docs/concepts/getting-started.md
• Added new sample project to the solution file

Testing

• All 49 OAuth tests pass across net8.0, net9.0, and net10.0
• New test CanAuthenticate_WithoutResourceIndicator validates the feature
• DI sample compiles and builds successfully