.NET: Switch auth sample to use Singletons

[westey (westey-m)]

Motivation and Context

#4446

Description

• Switch auth sample to use Singletons

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

Updates the AspNetAgentAuthorization end-to-end .NET sample to register the agent and its supporting services as singletons, aligning the sample with the recommended singleton-style registration pattern discussed in #4446.

Changes:

• Convert KeycloakUserContext to be singleton-safe by reading claims from the current HttpContext on-demand.
• Change DI registrations for IUserContext, ExpenseService, and AIAgent from scoped to singleton in the sample Program.cs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
dotnet/samples/05-end-to-end/AspNetAgentAuthorization/Service/UserContext.cs	Refactors user context to be singleton-friendly by resolving user info from HttpContext per property access.
dotnet/samples/05-end-to-end/AspNetAgentAuthorization/Service/Program.cs	Switches sample DI registrations to singletons for user context, expense service, and the agent.