Handle empty test failure stack traces

[kiwigitops]

Summary

• use InnerText when reading failure stack traces so empty XML failure nodes produce an empty string instead of $null
• keep the existing stack trace formatting for non-empty failures
• add a regression test with an empty <failure> node

Closes #2256.

Testing

• git diff --check
• Invoke-Pester -Path .\Tests\AnalyzeTests.Test.ps1 could not run locally because the installed Windows PowerShell Pester module is 3.4.0 and does not provide the repo helper's Add-AssertionOperator command.

[Copilot]

Pull request overview

Fixes the AnalyzeTests “normal” test result summarization to tolerate <failure> elements with no stack trace content (empty XML nodes), preventing runtime errors during markdown/HTML failure summary generation.

Changes:

• Switch stack trace extraction from ."#text" to .InnerText and reuse a normalized $stackTrace value for both log output and failure summary nodes.
• Add a Pester regression test that writes a minimal TestResults.xml containing an empty <failure> node and asserts GetTestResultSummaryMD does not throw.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Actions/AnalyzeTests/TestResultAnalyzer.ps1	Uses .InnerText to reliably read empty failure nodes as "" (not $null) and preserves existing formatting behavior for non-empty traces.
Tests/AnalyzeTests.Test.ps1	Adds regression coverage for empty <failure> stack traces by asserting GetTestResultSummaryMD runs without exceptions.

[aholstrup1]

Issue already being handled in #2257