Return deterministic empty .cratis identity introspection payloads when providers are absent

[Copilot]

Summary

Arc now maps the .cratis identity introspection endpoints even when identity/user/tenant providers are not configured, and returns deterministic empty payloads instead of falling through to host defaults (often index.html).

Added

• Added no-provider specs for identity schema, users, and tenants endpoint responses.
• Added assertions that provider-missing mapping still registers /.cratis/identity-details/schema, /.cratis/users, and /.cratis/tenants.

Changed

• Changed IdentityEndpointMapper to always map:
  • /.cratis/identity-details/schema
  • /.cratis/users
  • /.cratis/tenants
• Changed schema resolution to return {} when no IProvideIdentityDetails is registered.
• Changed users/tenants handlers to return [] when no provider instances are available.
• Updated identity endpoint handler test fixture to capture only the /.cratis/me handler, preserving existing identity request test behavior.
• Updated introspection docs to reflect always-mapped schema endpoint and empty-schema fallback.

var identityDetailsProvider = context.RequestServices.GetService<IProvideIdentityDetails>();
if (identityDetailsProvider is null)
{
    await context.WriteResponseAsJson(new JsonObject(), typeof(JsonObject), context.RequestAborted);
    return;
}

Fixed

• Fixed .cratis users/tenants/schema endpoints not being present in no-provider setups.
• Fixed no-provider responses to be explicit and machine-consumable ([] / {}) rather than implicit host fallback content.

Removed

• None.

Security

• None.

Deprecated

• None.