[0.84] Route validation npm requests through package proxy

[anuagragith]

Summary

• Routes npm/npx and Yarn package requests used by PR and Continuous validation through the anonymous PackageFeedProxy.
• Keeps Azure Artifacts authentication scoped to direct npm requests and the Verdaccio uplink.
• Prevents Yarn from sending the Azure feed token to PackageFeedProxy.

Motivation

A fresh validation run for #16465 includes the changes from #16467 and confirms that the original Setup failure is resolved.
However, the Fabric X64Hermes and X86Hermes jobs now fail while running:

npx --yes @react-native-community/cli codegen-windows --logging

npm returns an  E401  authentication error because later  npx  requests still use the authenticated Azure DevOps npm registry.

Using PackageFeedProxy for  NPM_CONFIG_REGISTRY  allows these PR and CI npm requests to resolve without depending on Azure DevOps feed credentials.

Release safety -
This change does not alter package publishing destinations:
• PR and Continuous pipelines consume  .ado/variables/shared.yml  through  build-template.yml .
•  release-pipeline.yml  does not import  shared.yml .
• npmjs publication uses the ESRP release task.
• Azure DevOps feed publication passes its destination explicitly using  npm publish --registry .
• The Release pipeline retains its own registry configuration for package-existence checks.

Validation:
• Resolved  @react-native-community/cli  through PackageFeedProxy using  npx ; returned version  20.2.0 .
• Prettier check passed for  .ado/variables/shared.yml .
•  git diff --check  passed.
• Confirmed the variable propagates to both PR and Continuous pipelines.
• Confirmed the Release pipeline does not consume the changed variable and publishing remains explicitly registry-scoped.

Related: #16465, #16467
 ###### Microsoft Reviewers: [Open in CodeFlow](https://microsoft.github.io/open-pr/?codeflow=https://github.com/microsoft/react-native-windows/pull/16480)

[Copilot]

🟡 Changes recommended

The network-isolation README still documents both registries as using react-native-public.

1 open finding

• Update package-feed documentation for split npm and Yarn registries · New

What changed in this PR

Routes validation-time npm/npx requests through PackageFeedProxy while keeping Yarn on the authenticated feed.

Changes:

• Updates NPM_CONFIG_REGISTRY to the anonymous proxy.
• Separates npm and Yarn routing documentation.

File	Description
.ado/​variables/​shared.yml	Configures split npm and Yarn registry routing.

🧠 Review effort: Balanced

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[Copilot]

🟢 Approval recommended

The focused configuration and documentation changes are consistent with validation and release pipeline behavior.

1 open finding

• Update package-feed documentation for split npm and Yarn registries

🧠 Review effort: Balanced

[Copilot]

🟡 Changes recommended

Yarn routing and authentication contradict the PR’s explicit requirement to keep Yarn on the existing feed.

1 open finding

• Keep Yarn on Azure Artifacts and retain CI authentication · New

1 resolved since last review

• Update package-feed documentation for split npm and Yarn registries

🧠 Review effort: Balanced