Add GitHub Action workflow to publish NuGet packages

[Copilot]

Automates NuGet package publishing via manual workflow dispatch, replacing manual command-line publishing.

Changes

• New workflow .github/workflows/publish-nuget.yml

  • Manual trigger with version input parameter
  • Builds package using existing Azure code signing infrastructure
  • Publishes to NuGet.org with --skip-duplicate flag
  • Uploads artifact for reference
  • Explicit permissions: contents: read, id-token: write

• README documentation

  • Added "Publishing a New Release" section with workflow execution steps
  • Notes on semantic versioning requirements

Required Secret

NUGET_API_KEY must be configured in repository settings:

1. Generate API key at nuget.org with push permissions for EZRadiusClient
2. Add to Settings → Secrets and variables → Actions
3. Name: NUGET_API_KEY

The existing EZRADIUS_CLIENT_DEPLOYER secret handles Azure code signing.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Create GitHub Action to Publish NuGet Package</issue_title>
<issue_description>## Current State

Today, pushing new versions of the NuGet package require manually running commands from a user's machine.

Desired State

A GitHub Action with a manual workflow dispatch can publish a new version of the NuGet package for EZRadiusClient. Just run the workflow dispatch and the workflow will take care of building and publishing

Requirements

• The GitHub Action will be triggered manually
• The README is updated with steps on how to run a release
• The PR description is updated with steps for how to retrieve and configure secrets that are to be used by the Action.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Create GitHub Action to Publish NuGet Package #5

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