Fix issues in GetKeyedService() and GetKeyedServices() with AnyKey

[steveharter]

Reviving the closed v9.0 PR #97561:

• (in previous PR) Fix consistency issue related to a descriptor cache bug.
• (in previous PR) Do not return AnyKey registration when enumerating services with AnyKey as the lookup key.
• (new) Throw when attempting to resolve a single service with AnyKey.

Semantics here mostly related to fallout and discussions from #95582. Here's a summary of IEnumerable-based queries that was copied from a test that was added in this PR:

/*
 * Table for what results are included in GetServices()\GetKeyedServices():
 *
 * Query                     | Keyed? | Unkeyed? | AnyKey? | nullkey?
 * -------------------------------------------------------------------
 * GetServices(Type)         | no     | yes      | no      | yes
 * GetKeyedServices(null)    | no     | yes      | no      | yes
 * GetKeyedServices(AnyKey)  | yes    | no       | no      | no
 * GetKeyedServices(key)     | yes    | no       | no      | no
 *
 * Summary:
 * - A null key is the same as unkeyed. This allows the KeyServices APIs to support both keyed and unkeyed.
 * - AnyKey is a special case of Keyed.
 * - It is not possible to query for AnyKey registrations using IEnumerable; a singleton query resolves.
 */

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PR Overview

This PR addresses inconsistencies in the resolution and caching behaviors for KeyedService.AnyKey. It updates the logic in service call site creation, modifies the service provider methods to validate AnyKey usage, and revises tests to ensure the correct behavior and exception throwing.

Reviewed Changes

File	Description
ServiceLookup/CallSiteFactory.cs	Adjusted the keys matching logic and caching behavior to exclude AnyKey registrations while preserving proper lookups.
Specification.Tests/KeyedDependencyInjectionSpecificationTests.cs	Updated tests to reflect the revised expectations for service counts and exception throwing regarding AnyKey usage.
ServiceProvider.cs	Added explicit checks to throw exceptions when AnyKey is used to resolve a non-collection service.
ServiceLookup/ThrowHelper.cs	Introduced a helper method to throw an InvalidOperationException for invalid AnyKey resolution attempts.

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.