[PWGDQ] make qaMatching AO2D index merger-safe

[nivram-phy]

Summary

• make the qaMatching event-reference column use a merger-safe custom index label
• preserve canonical AO2D tree naming for the qaMatching output tables

Problem

The previous qaMatching AO2D output produced an event-reference branch that was not resolved correctly by o2-aod-merger and caused downstream Hyperloop issues.

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 2 warnings, 🔕 0 disabled

[nivram-phy]

@aferrero2707 This is the new PR with the unbounded index issue resolved (hopefully). I chcked it locally with the o2-aod-merger