Avoid stack overflow flattening test results

[danmoseley]

Note

This PR description was drafted with GitHub Copilot assistance.

Fixes #1179

Avoid stack overflow when flattening large result groups in testplan_show_test_results_from_build_id.

The previous implementation used:

allResults.push(...group.results)

For very large result groups, that spreads every result as a function argument and can exceed JavaScript engine argument limits, throwing Maximum call stack size exceeded:

Tool: ado-dnceng-public-testplan_show_test_results_from_build_id
Args: { "project": "public", "buildid": 1395596 }
Result: MCP server 'ado-dnceng-public': Error fetching test results: Maximum call stack size exceeded

This PR changes the flattening to iterate results and push one item at a time. The structure of the JSON is not changed -- it's the same structure the code would produce if it didn't stack overflow the engine by using spread.

GitHub issue number

#1179

Associated Risks

This is intended to be behavior-preserving except for avoiding the spread/argument-limit failure. It does not otherwise change result fetching or output shape.
Returning very large output successfully may now expose a limitation elsewhere.

✅ 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/test-plan.test.ts --runInBand
• npm run validate-tools

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1183   +/-   ##
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  Coverage        ?   90.25%           
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  Files           ?       19           
  Lines           ?     2125           
  Branches        ?      498           
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  Hits            ?     1918           
  Misses          ?      111           
  Partials        ?       96           

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

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