Add "envvar" authentication option

[kyle-rader-msft]

Support passing in a token via environment variable

#608

Associated Risks

There are very few risks here, as taking in an EntraID token inherently means another tool has already handled authentication and we now have a short-lived token.

✅ PR Checklist

• I have read the contribution guidelines
• I have read the code of conduct guidelines
• Title of the pull request is clear and informative.
• 👌 Code hygiene
• 🔭 Telemetry - N/A
• 📄 Documentation: CLI Help Text, Getting Started, and FAQ Updated.
• 🛡️ N/A -

🧪 How did you test it?

I've manually tested by

• using azureauth ado token to acquire an ADO access token
• export it as an env var and launch the MCP server via

  ADO_MCP_AUTH_TOKEN=$(azureauth ado token) npm start -- Office -a envvar
  

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@4c0f9d0). Learn more about missing BASE report.

Additional details and impacted files

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Flag	Coverage Δ
unittests	96.47% <ø> (?)

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[danhellem]

Sorry @kyle-rader-msft but we need folks to follow the process outlined in https://github.com/microsoft/azure-devops-mcp/blob/main/CONTRIBUTING.md

Please create an Issue first before creating a PR. I will going to close this PR for the time being.

[danhellem]

@aaudzei assigning to you to review per Teams chat