Add a static config accessor that can help with migration

[twsouthwick]

This enables switching from ConfigurationManager.[Appsettings|ConnectionStrings] to AppConfiguration.[GetSetting|GetConnectionString] so that there is a static configuration source on all platforms while adapting to more modern practices of dependency injection for configuration.

[joperezr]

Do we need to make a remark on the requirements of the string parameter? Pretty sure there are keys that are not supported so I wonder if we should either do some sanitization or document we assume it is a valid key.

[twsouthwick]

I don't know of any IConfiguration[index] constraints. If there are, we can document it but it's nothing that doesn't already exist there as this is just a pass through since we don't want to add the dependency to IConfiguration for consumers of the Microsoft.AspNetCore.SystemWebAdapters package

[joperezr]

Left a few minor comments but looks good otherwise

[Copilot]

Pull Request Overview

Adds a static AppConfiguration API and supporting infrastructure to enable migration from ConfigurationManager to IConfiguration across Core and Framework targets. Introduces an internal IConfigurationAccessor service and registers it during system web adapter setup; provides samples and end-to-end tests validating behavior; adjusts host startup to ensure services are available before returning.

• Introduces AppConfiguration static helper and IConfigurationAccessor interface.
• Registers configuration accessor in Core and Framework paths and adds samples (Core, Framework, AppHost) plus E2E tests.
• Modifies host builder to wait for application start before returning.

Reviewed Changes

Copilot reviewed 32 out of 35 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
test/Microsoft.AspNetCore.SystemWebAdapters.E2E.Tests/Microsoft.AspNetCore.SystemWebAdapters.E2E.Tests.csproj	Adds project reference to new AppConfig sample for E2E testing.
test/Microsoft.AspNetCore.SystemWebAdapters.E2E.Tests/AppConfigTests.cs	New tests validating static configuration accessor across Core and Framework apps.
src/Services/ConfigurationAccessorExtensions.cs	Adds internal extension to register IConfigurationAccessor implementation.
src/Microsoft.AspNetCore.SystemWebAdapters/Microsoft.AspNetCore.SystemWebAdapters.csproj	Includes new source files for AppConfiguration and IConfigurationAccessor; adds InternalsVisibleTo for FrameworkServices.
src/Microsoft.AspNetCore.SystemWebAdapters/AppConfiguration.cs	Adds static AppConfiguration API with XML docs.
src/Microsoft.AspNetCore.SystemWebAdapters/Adapters/IConfigurationAccessor.cs	Defines internal IConfigurationAccessor interface.
src/Microsoft.AspNetCore.SystemWebAdapters.FrameworkServices/SystemWebAdapterConfiguration.cs	Registers configuration accessor during adapter setup.
src/Microsoft.AspNetCore.SystemWebAdapters.FrameworkServices/Hosting/HttpApplicationHostBuilder.cs	Waits for application start using TaskCompletionSource after queuing host run.
src/Microsoft.AspNetCore.SystemWebAdapters.FrameworkServices/Hosting/HttpApplicationHost.cs	Registers configuration accessor in host creation.
src/Microsoft.AspNetCore.SystemWebAdapters.FrameworkServices/Hosting/ConfigurationManagerConfigExtensions.cs	Refactors enumeration of connection strings and app settings to strongly typed loops.
src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/SystemWebAdaptersExtensions.cs	Registers configuration accessor in Core services setup.
samples/AppConfig/... (multiple)	Adds Core and Framework sample apps plus AppHost to demonstrate configuration migration.
Microsoft.AspNetCore.SystemWebAdapters.sln	Adds solution folder and projects for new AppConfig samples.

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