Integrate BlazorAuthenticationChallengeHandler and login/logout endpoint helpers into Microsoft.Identity.Web package

[jmprieur]

Goal

Integrate BlazorAuthenticationChallengeHandler and LoginLogoutEndpointRouteBuilderExtensions from .github/skills/entra-id-aspire-authentication into the main Microsoft.Identity.Web NuGet package.

• Add these files to a new src/Microsoft.Identity.Web/Blazor/ subfolder.
• Adapt doc comments to clarify Blazor Server usage.
• Add them to the main csproj.
• Unit test main behaviors (incremental consent/challenge handler, endpoint behaviors).
• Add a new minimal Aspire-based DevApp under tests/DevApps/AspireBlazorCallsWebApi/, wired to use the same app registrations/credentials as WebAppCallsWebApi DevApp.
• Demonstrate integration in aspire.md (sample code, UI flow).

Important: Do NOT update SKILL.md or doc instructions referencing copy from skill folder yet. That will be handled post-release.

Testing

• Unit tests for helper classes
• Manual verification of end-to-end flow in new Aspire DevApp with re-used tenant credentials

Tracking

• This issue tracks ONLY the integration into the main codebase. Separate issue will be opened for docs/skill updates post-release.

/cc @jmprieur