feat: add keyed Dataverse client registration

[Copilot]

This PR adds named Dataverse client registration on top of the existing AddDataverseClient() API using native keyed DI, enabling multiple independently configured clients (for example source/target) in the same app. The unnamed registration path remains unchanged and isolated from the named path.

• API surface: named AddDataverseClient overloads

  • Added:
    • AddDataverseClient(string name, Action<DataverseClientOptions> configureOptions)
    • AddDataverseClient(string name, IConfiguration configuration)
  • Both configure named options and route through a dedicated named registration path.

• DI wiring: separate keyed registration path

  • Added a separate private AddNamedDataverseClientCore(string name) in ServiceCollectionExtensions.
  • Registers:
    • keyed singleton ServiceClient
    • keyed scoped IOrganizationServiceAsync2 via Clone()
  • No reuse of the unnamed core path, preventing default-name collisions.

• Factory support for named options

  • Added ServiceClientFactory.CreateNamed(IServiceProvider, string name).
  • Uses IOptionsMonitor<DataverseClientOptions>.Get(name) to resolve named options.
  • Keeps existing Create(IServiceProvider) unchanged.

• Test coverage for named behavior

  • Added tests for:
    • keyed service registrations/lifetimes
    • named options binding (action + configuration)
    • chaining return contract
    • coexistence of named + unnamed registrations
    • coexistence of two named clients
    • named validation failures (missing URL / non-HTTPS)
    • ServiceClientFactory.CreateNamed behavior (default/custom credential, deferred connection)

Example usage:

services.AddDataverseClient("source", options =>
{
    options.OrganizationUrl = new Uri("https://source.crm4.dynamics.com");
    options.DeferConnection = true;
});

services.AddDataverseClient("target", configuration.GetSection("Dataverse:Target"));

// consumer
public sealed class Migrator(
    [FromKeyedServices("source")] IOrganizationServiceAsync2 source,
    [FromKeyedServices("target")] IOrganizationServiceAsync2 target)
{
}