Handle missing pull request thread arrays

[danmoseley]

Note

This PR description was drafted with GitHub Copilot assistance.

Fixes #1180

Normalize missing pull request thread responses in repo_list_pull_request_threads.

If gitApi.getThreads(...) returns undefined, JSON.stringify(undefined) also returns undefined, which can produce an invalid MCP tool result because content[0].text must be a string, from Copilot here:

Tool: ado-dnceng-repo_list_pull_request_threads
Args: { "repositoryId": "copilot-agent-runtime", "project": "copilot-agent-runtime", "pullRequestId": 6536, "status": "Active" }
Result: Invalid tools/call result ... content[0].text expected string, received undefined

This PR normalizes missing thread responses to [] and keeps the rest of the filtering/paging/mapping logic array-based.

GitHub issue number

#1180

Associated Risks

Low. The intended behavior for a missing thread array is to return an empty JSON array rather than an invalid MCP result shape.

✅ 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 added, updated, or N/A
• 📄 Documentation added, updated, or N/A
• 🛡️ Automated tests added, or N/A

🧪 How did you test it?

• npm test -- --runTestsByPath test/src/tools/repositories.test.ts --runInBand
• npm run validate-tools

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@546e37d). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1184   +/-   ##
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  Coverage        ?   90.25%           
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  Files           ?       19           
  Lines           ?     2124           
  Branches        ?      499           
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  Hits            ?     1917           
  Misses          ?      111           
  Partials        ?       96           

Flag	Coverage Δ
unittests	90.25% <100.00%> (?)

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[danmoseley]

example of impact in Copilot CLI session, I just got it again--