fix: handle single summary artifacts

[mkolasinski-splunk]

What changed

• Download test-summary artifacts into a dedicated summaries/ directory.
• Discover job_summary.txt recursively instead of assuming every artifact is extracted into a named subdirectory.
• Apply the fix consistently to knowledge, UI, modinput, UCC modinput, upgrade, scripted-input, and SPL2 integration report jobs.
• Emit an explicit error when no summary files were downloaded.

Root cause

When actions/download-artifact@v8 receives a pattern matching a single artifact, its contents can be extracted directly into the configured destination. The existing report steps expected paths such as summary-ko*/job_summary.txt, so a one-item test matrix failed in the report aggregation even when its underlying tests passed.

This was observed in the Carbon Black v2.21.3 rollout: the Splunk 10.4.2 knowledge suite passed, but knowledge-tests-report failed with cat: 'summary-ko*/job_summary.txt': No such file or directory.

Impact

Single-artifact and multi-artifact test matrices now generate the same report successfully. The explicit missing-artifact error also makes genuine download failures easier to diagnose.

Validation

• YAML parsed successfully with yq.
• yamlfmt -lint passed.
• git diff --check passed.
• Extracted shell for all seven report jobs passed bash -n and ShellCheck.
• Semantic fixtures passed for single-artifact and multi-artifact directory layouts.
• Missing-artifact fixture exited with the expected explicit error.

Full repository actionlint still reports unrelated pre-existing errors on develop involving actions/create-github-app-token@v3 inputs and an existing matrix expression.