.NET: Prefer HTTPS Aspire DevUI backends

[tommasodotNET]

Motivation & Context

Aspire DevUI integration currently resolves agent service backends through the hard-coded HTTP endpoint. When an agent service enables HTTPS redirection, the DevUI proxy forwards the redirect response back to the browser, which bypasses the DevUI prefix routing and breaks requests to the backend.

This change lets the DevUI integration work with agent services that expose both HTTP and HTTPS endpoints and use HTTPS redirection.

Description & Review Guide

• What are the major changes?
  • DevUI backend resolution now prefers an allocated https endpoint and falls back to http for existing services.
  • Added unit tests covering HTTPS preference, HTTP fallback when HTTPS is missing, and HTTP fallback when HTTPS exists but is not allocated.
  • Updated the DevUI Aspire integration sample so WriterAgent launches with an HTTPS profile and uses HTTPS redirection, providing a manual test path for this scenario.
• What is the impact of these changes?
  • Agent services configured with HTTPS redirection can be used through Aspire DevUI without leaking raw redirect responses to the browser.
  • Existing HTTP-only agent services continue to work through the fallback behavior.
• What do you want reviewers to focus on?
  • Whether preferring https over http is the right default for DevUI backend resolution, and whether the sample is a useful enough regression scenario for manual validation.

Related Issue

Fixes #6769

No other open PR was found for this issue.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Pull request overview

This PR fixes Aspire DevUI proxying for agent backends that enable UseHttpsRedirection() by updating backend URL resolution to prefer an allocated HTTPS endpoint (falling back to HTTP), preventing redirect responses from leaking to the browser and bypassing DevUI prefix routing.

Changes:

• Updated DevUI backend resolution to prefer https then fall back to http, only using allocated endpoints.
• Added unit tests for HTTPS preference and HTTP fallback scenarios (missing/unallocated HTTPS).
• Updated the DevUI Aspire integration sample to launch WriterAgent with an HTTPS profile and enable HTTPS redirection for manual verification.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
dotnet/tests/Aspire.Hosting.AgentFramework.DevUI.UnitTests/DevUIAggregatorHostedServiceTests.cs	Adds coverage for backend endpoint selection (HTTPS preferred; HTTP fallback).
dotnet/src/Aspire.Hosting.AgentFramework.DevUI/DevUIAggregatorHostedService.cs	Implements preferred endpoint selection (https → http) and exposes resolution for testing.
dotnet/samples/05-end-to-end/DevUIAspireIntegration/WriterAgent/Properties/launchSettings.json	Adds an https launch profile that exposes both HTTPS and HTTP URLs.
dotnet/samples/05-end-to-end/DevUIAspireIntegration/WriterAgent/Program.cs	Enables UseHttpsRedirection() to reproduce and validate the redirect scenario.
dotnet/samples/05-end-to-end/DevUIAspireIntegration/README.md	Documents that the sample’s WriterAgent uses HTTPS redirection for DevUI validation.
dotnet/samples/05-end-to-end/DevUIAspireIntegration/DevUIIntegration.AppHost/Program.cs	Launches WriterAgent via the https profile and targets the HTTPS endpoint for health checks.