Add development users/tenants discovery endpoints on /.cratis

[Copilot]

Summary

Adds development-only provider discovery for suggested users and tenants, exposed as /.cratis/users and /.cratis/tenants for tooling integration.

Added

• ICanProvideUsers and User (MicrosoftIdentity: ClientPrincipal, Details: object) in Cratis.Arc.Identity.
• ICanProvideTenants, Tenant (Id: TenantId, Name: TenantName), and TenantName concept in Cratis.Arc.Tenancy.

Changed

• IdentityEndpointMapper now discovers ICanProvideUsers / ICanProvideTenants implementations via ITypes.
• Maps /.cratis/users and /.cratis/tenants only when corresponding providers are present.
• Aggregates results across all discovered providers before writing JSON response.

public interface ICanProvideUsers
{
    Task<IEnumerable<User>> Provide();
}

public interface ICanProvideTenants
{
    Task<IEnumerable<Tenant>> Provide();
}

Fixed

• Arc now exposes explicit development endpoints for available users and tenants, matching the required /.cratis contract for downstream tooling.

Removed

• None.

Security

• No auth model changes to existing identity flow; new endpoints are discovery-driven and only exposed when providers are implemented.

Deprecated

• None.

[einari]

@copilot drop the cancellationtoken as part of the interfaces

[github-actions]

NuGet packages for this PR, e.g. Cratis.Arc:
https://github.com/cratis/arc/packages/1655206?version=20.26.19-pr2250.fa3d32e

[Copilot]

@copilot drop the cancellationtoken as part of the interfaces

Fixed in d9a339e — removed CancellationToken from ICanProvideUsers and ICanProvideTenants, and updated endpoint mapper/spec provider implementations accordingly.

[einari]

@copilot Leverage the IInstancesOf<> for discovery, rather than using the ITypes interface directly.

[github-actions]

NuGet packages for this PR, e.g. Cratis.Arc:
https://github.com/cratis/arc/packages/1655206?version=20.26.19-pr2250.d9a339e

[Copilot]

@copilot Leverage the IInstancesOf<> for discovery, rather than using the ITypes interface directly.

Done in 9fec480 — switched users/tenants discovery to IInstancesOf<> and updated the IdentityEndpointMapper specs accordingly.

[github-actions]

NuGet packages for this PR, e.g. Cratis.Arc:
https://github.com/cratis/arc/packages/1655206?version=20.26.19-pr2250.9fec480

[einari]

@copilot There is a regression: https://github.com/Cratis/Arc/actions/runs/26438008872/job/77841851283?pr=2250

[Copilot]

@copilot There is a regression: https://github.com/Cratis/Arc/actions/runs/26438008872/job/77841851283?pr=2250

Addressed in 4787d75. I fixed a race condition in XmlDocumentation by synchronizing access to the static XML document cache, which stabilizes the failing ProxyGenerator XML documentation spec in CI.

[github-actions]

NuGet packages for this PR, e.g. Cratis.Arc:
https://github.com/cratis/arc/packages/1655206?version=20.26.19-pr2250.4787d75