Fix download issues in pipelines

[vmoroz]

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Automation (AI changes or Github Actions to reduce effort of manual tasks)

Why

Our CI pipelines suffer from two recurring issues:

1. yarn validate-overrides hits GitHub's unauthenticated API rate limit (60 req/hr),
   causing intermittent linting failures.
2. .NET Core Hosting Bundle and SDK downloads in Desktop builds fail due to transient
   network errors (Invoke-WebRequest: The response ended prematurely), breaking
   ARM64EC and other Desktop jobs.

Additionally, the platform-override-zero-permission-token variable group must be
removed per new security requirements.

What

• Linting job: Added AzureKeyVault@2 task to fetch a no-permission GitHub PAT
  (gitHubApiAuthToken) from rnw-keyvault and pass it as
  PLATFORM_OVERRIDE_GITHUB_TOKEN to yarn validate-overrides. Uses different service
  connections for PR (CXE - RNW CI & Anomaly-Detection) and CI
  (Office-React-Native-Windows-Bot).
• PR pipelines: Removed platform-override-zero-permission-token variable group
  from pr-pipeline.yml and windows-vs-pr.yml.
• Desktop builds: Added Invoke-WebRequestWithRetry helper (3 attempts with
  incremental backoff) for .NET hosting bundle and SDK downloads in
  desktop-single.yml.

Screenshots

N/A — pipeline YAML changes only.

Testing

• PR pipeline Linting job passes and yarn validate-overrides no longer hits rate limits
• CI pipeline Linting job passes with Office-React-Native-Windows-Bot service connection
• Desktop build jobs still install .NET hosting bundle and SDK successfully
• Removal of platform-override-zero-permission-token variable group does not break any pipeline

Changelog

Should this change be included in the release notes: no

Microsoft Reviewers: Open in CodeFlow