Fix memory leak: RuntimeConfigProvider._changeTokenRegistration never disposed

[aaronburtle]

Why make this change?

Closes #3403

What is this change?

RuntimeConfigProvider has a Dispose() method that cleans up its _changeTokenRegistration (a ChangeToken.OnChange subscription), but this method was never called due to two compounding issues:

• The class did not implement the IDisposable interface it only had a public Dispose() method. The DI container only auto-disposes services that implement IDisposable or IAsyncDisposable.

• It was registered as a pre-created instance via services.AddSingleton(configProvider). The .NET DI container does not dispose pre-created instances; only instances it creates itself via type or factory registrations.

We also correctly register the loader in our tests using the same pattern (Loader was correctly registered in Startup).

How was this tested?

Run against our test suite.

[Copilot]

Pull request overview

Fixes a resource leak in the service startup/config hot-reload path by ensuring RuntimeConfigProvider’s change-token registration is properly disposed when the host/service provider is torn down.

Changes:

• Make RuntimeConfigProvider implement IDisposable and guard against double-dispose while disposing the ChangeToken.OnChange registration.
• Update DI registrations to use singleton factory registrations (AddSingleton<T>(sp => instance)) so the container owns and disposes the singleton.
• Align affected test setups to use the same DI registration pattern.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Service/Startup.cs	Registers RuntimeConfigProvider via factory so DI disposes it at shutdown.
src/Core/Configurations/RuntimeConfigProvider.cs	Implements IDisposable and disposes _changeTokenRegistration.
src/Service.Tests/SqlTests/SqlTestBase.cs	Updates test DI registration to container-owned singleton factory.
src/Service.Tests/Mcp/McpQueryTimeoutTests.cs	Updates test DI registration to container-owned singleton factory.
src/Service.Tests/Mcp/EntityLevelDmlToolConfigurationTests.cs	Updates test DI registration to container-owned singleton factory.
src/Service.Tests/Mcp/DescribeEntitiesFilteringTests.cs	Updates test DI registration to container-owned singleton factory.
src/Service.Tests/Mcp/AggregateRecordsToolTests.cs	Updates test DI registration to container-owned singleton factory.
src/Service.Tests/CosmosTests/TestBase.cs	Updates test DI registrations for loader/provider to factory form for disposal.
src/Service.Tests/Authentication/JwtTokenAuthenticationUnitTests.cs	Updates test DI registration to container-owned singleton factory.
src/Service.Tests/Authentication/Helpers/WebHostBuilderHelper.cs	Updates test DI registrations to container-owned singleton factory.