fix: QA index processor preview key error

[cyphercodes]

Summary

Fixed a KeyError that occurred when using QA blocks in knowledge base workflows.

Root Cause

The QAIndexProcessor.format_preview method was returning a dictionary with the key 'qa_preview', but the parent IndexProcessor.format_preview method expected the key 'preview'. This caused a KeyError: 'preview' when iterating through the preview items.

Changes

• api/core/rag/index_processor/processor/qa_index_processor.py: Changed the return key from 'qa_preview' to 'preview' to match the expected interface.

Testing

This fix ensures that QA blocks in knowledge base workflows can be previewed without errors.

Fixes #34144

[mahdirajaee]

Clean, minimal fix that addresses the root cause directly. The QA index processor was returning qa_preview while the base IndexProcessor.format_preview interface expects preview, so this KeyError would have occurred every time a user attempted to preview QA-indexed segments through the knowledge base workflow.

One concern worth considering: this is a public-facing API response key change. If any external consumers (plugins, third-party integrations, or the frontend) were already handling the qa_preview key as a fallback or special case, changing it to preview could break those consumers in a different way. It would be worth verifying that the frontend chunk preview rendering code only reads from preview and never checks for qa_preview. That said, since the bug report indicates this was simply broken (KeyError), it is likely nobody was successfully consuming the old key anyway. The fix looks correct.