[CI Failure] NPM registry rate limiting (HTTP 429 Too Many Requests) from dnceng feed

## Build Information

Build: https://github.com/microsoft/aspire/actions/runs/28948977829
Build error leg: Tests / Run VS Code extension unit tests (Windows)
Pull request: #18688

## Error Message

```
error Error: https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public-npm/npm/registry/whatwg-encoding/-/whatwg-encoding-3.1.1.tgz: Request failed "429 Too Many Requests"
at ResponseError.ExtendableBuiltin
at new ResponseError
```

## Description

NPM registry rate limiting (HTTP 429 Too Many Requests) from dnceng feed

**Type**: infra-failure

## Occurrences

| Date | Build | Job | PR |
|------|-------|-----|----|
| 2026-07-08 | [28948977829](https://github.com/microsoft/aspire/actions/runs/28948977829) | Tests / Run VS Code extension unit tests (Windows) | #18688 |

Start with the linked Actions run and its Tests / Run VS Code extension unit tests (Windows) job, then inspect the workflow and npm registry configuration used by that job. Reproduce the dependency install and consider the issue complete when the Windows unit-test leg finishes without HTTP 429 failures from the dnceng feed.

Written by the indexing model from the issue text.