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.

After this PR is in, I'll update push another PR to update the docs around IEnumerable semantics.

Semantics here mostly related to fallout and discussions from #95582. Here's a summary of semantics copied from a test added in this PR:

/*
 * Table for what results are included:
 *
 * Query                     | Keyed? | Unkeyed? | AnyKey? | null key?
 * -------------------------------------------------------------------
 * GetServices(Type)         | no     | yes      | no      | yes
 * GetService(Type)          | no     | yes      | no      | yes
 *
 * GetKeyedServices(null)    | no     | yes      | no      | yes
 * GetKeyedService(null)     | no     | yes      | no      | yes
 *
 * GetKeyedServices(AnyKey)  | yes    | no       | no      | no
 * GetKeyedService(AnyKey)   | throw  | throw    | throw   | throw
 *
 * GetKeyedServices(key)     | yes    | no       | no      | no
 * GetKeyedService(key)      | yes    | no       | yes     | 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.
 * - AnyKey registrations are not returned with GetKeyedServices(AnyKey) and GetKeyedService(AnyKey) always throws.
 * - For IEnumerable, the ordering of the results are in registration order.
 * - For a singleton resolve, the last match wins.
 */

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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.

[halter73]

LGTM. I think having GetKeyedServices<T>(KeyedService.AnyKey) return all registrations for each specific key rather than the last one is fine. I was just curious if @stephentoub had any opinions since he first asked for AnyKey to even work with GetKeyedServices.

It would still be nice if we could assert the right order is maintained for the services returned by GetKeyedServices in the QueryWithIEnumerable test. I don't think the order is wrong, but it'd be nice to veriffy.