[CI Failure] Set up .NET Core step fails with 'read ECONNRESET' network error

## Build Information

Build: https://github.com/microsoft/aspire/actions/runs/34432786645
Build error leg: Tests / Package tests - macOS (class, Aspire.Templates.Tests, Templates-XUnit_V3MTP_NewUpAndBuildSupportP... / Templates-XUnit_V3MTP_NewUpAndBuildSupportProjectTemplatesTests (macos-latest)
Pull request: #19999

## Error Message

```
read ECONNRESET (occurs during 'Set up .NET Core' step, likely while downloading the SDK/runtime; transient network reset unrelated to PR content)
```

## Description

Set up .NET Core step fails with 'read ECONNRESET' network error

**Type**: infra-failure

## Occurrences

| Date | Build | Job | PR |
|------|-------|-----|----|
| 2026-09-10 | [34432786645](https://github.com/microsoft/aspire/actions/runs/34432786645) | Tests / Package tests - macOS (class, Aspire.Templates.Tests, Templates-XUnit_V3MTP_NewUpAndBuildSupportP... / Templates-XUnit_V3MTP_NewUpAndBuildSupportProjectTemplatesTests (macos-latest) | #19999 |
| 2026-09-14 | [34790579218](https://github.com/microsoft/aspire/actions/runs/34790579218) | Tests / No-package tests (regular, Aspire.Cli.Tests, Cli, Cli, tests/Aspire.Cli.Tests/Aspire.Cli.Tests.cs... / Cli (macos-latest) | #20079 |
| 2026-09-16 | [35043367059](https://github.com/microsoft/aspire/actions/runs/35043367059) | Tests / Package tests - Linux (class, Aspire.Templates.Tests, Templates-NUnit_NewUpAndBuildSupportProject... / Templates-NUnit_NewUpAndBuildSupportProjectTemplatesTests (ubuntu-latest) | #20140 |
| 2026-09-16 | [35125246087](https://github.com/microsoft/aspire/actions/runs/35125246087) | Tests / Package tests - Windows (class, Aspire.Templates.Tests, Templates-NUnit_NewUpAndBuildSupportProje... / Templates-NUnit_NewUpAndBuildSupportProjectTemplatesTests (windows-latest) | #20171 |

Start by comparing the listed GitHub Actions runs and locate the “Set up .NET Core” step across the affected macOS, Linux, and Windows jobs. Determine whether ECONNRESET is reproducible or transient; done requires an agreed CI mitigation or evidence that the setup step is stable across the affected jobs.

Written by the indexing model from the issue text.