Correct keyed-service probing for built-in DI services

[Copilot]

IServiceProviderIsKeyedService was reporting built-in DI service types as available for arbitrary keys, while actual keyed resolution correctly failed. This change aligns probing with resolvability by treating built-ins as available only for the null key.

• Behavior fix (CallSiteFactory.IsService)

  • Restrict built-in service fallback (IServiceProvider, IServiceScopeFactory, IServiceProviderIsService, IServiceProviderIsKeyedService) to serviceKey is null.
  • Preserve existing descriptor/AnyKey lookup behavior for explicitly registered keyed services.

• Regression coverage (spec tests)

  • Add theory coverage for all four built-in service types.
  • Assert:
    • IsKeyedService(type, null) == true
    • IsKeyedService(type, nonNullKey) == false
    • IsKeyedService(type, KeyedService.AnyKey) == false

// New expected behavior for built-ins:
Assert.True(spiks.IsKeyedService(typeof(IServiceProvider), null));
Assert.False(spiks.IsKeyedService(typeof(IServiceProvider), "foo"));
Assert.False(spiks.IsKeyedService(typeof(IServiceProvider), KeyedService.AnyKey));

Fixes #127537

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[Copilot]

Pull request overview

This PR fixes keyed-service probing in the default Microsoft.Extensions.DependencyInjection provider so built-in DI services are reported as keyed-resolvable only when the service key is null, matching actual resolution behavior.

Changes:

• Gates built-in DI service fallback in CallSiteFactory.IsService on ServiceKey is null.
• Adds specification regression coverage for the four built-in DI service types with null, non-null, and KeyedService.AnyKey probes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/Microsoft.Extensions.DependencyInjection/src/ServiceLookup/CallSiteFactory.cs	Aligns built-in service probing with keyed resolution semantics.
src/libraries/Microsoft.Extensions.DependencyInjection.Specification.Tests/src/ServiceProviderIsKeyedServiceSpecificationTests.cs	Adds regression coverage for keyed probing of built-in DI services.