Publish .NET SDK packages to the internal feed

[MackinnonBuck]

Summary

Add a publish-dotnet-internal job to mirror GitHub.Copilot.SDK to the copilot-canary Azure Artifacts NuGet feed alongside the existing internal npm publication.

• Reuse the dotnet-package artifact produced by the public publishing job; do not rebuild the package.
• Run after successful public NuGet publication for non-unstable releases from main.
• Reuse the existing cicd environment, id-cpd-ci OIDC identity, and Azure DevOps resource. Pass the short-lived token through NuGet's source-credentials environment variable rather than storing it in a config file or command-line arguments.
• Reject unexpected feed URLs and package counts, skip duplicate package versions, and leave .snupkg publication on NuGet.org.

Existing npm publication and public release jobs are unchanged.

Validation

• Parsed the workflow YAML and checked Bash syntax.
• Confirmed artifact reuse, release gating, OIDC settings, and that all existing jobs are unchanged.
• Exercised nine mocked publication scenarios: a package plus symbols, a path containing spaces, missing packages, symbols only, multiple packages, an incorrect destination, and failures in authentication, source configuration, or publication.
• Live Azure OIDC authentication and internal-feed publication have not been exercised; no publishing workflow was triggered.

[Copilot]

Copilot review overview

🔵 Needs a closer look

The release workflow and external OIDC feed integration require human validation because live authentication and publication remain untested.

Review tier: Balanced
Findings: None

What changed in this PR

Adds internal Azure Artifacts publication for the .NET SDK after successful public NuGet publication.

Changes:

• Reuses the existing dotnet-package artifact.
• Authenticates through Azure OIDC and environment-based NuGet credentials.
• Validates feed URL/package count and skips duplicate versions.

File	Description
.github/​workflows/​publish.yml	Adds the guarded internal .NET publishing job.

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