Fix generation error precedence in failure classification

[edknv]

Description

NVBug 6622224.

Batch Live RAG evaluation currently classifies generic generation failures such as transport_error and request_error as judge_error when generation produces an empty candidate and no judge score is available. This incorrectly attributes the failure to the judge stage even though gen_error identifies generation as the originating stage.

This change gives generation errors precedence when deriving failure_mode. The existing thinking_truncated classification remains unchanged, while other non-null generation errors now use the stable generation_error bucket. judge_error is reserved for cases where generation succeeded but judging did not produce a score. The DataFrame scoring path also normalizes missing generation-error values before classification.

This affects only batch/operator evaluation and reporting. Single-query Retriever.answer() already returns before judging or scoring after a generation failure.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[greptile-apps]

Greptile Summary

This PR updates batch RAG evaluation to prioritize generation failures over missing judge scores and normalizes missing generation-error values in DataFrame scoring.

• Maps non-null generation errors to the stable generation_error failure bucket while preserving thinking_truncated.
• Adds pipeline-level coverage for generation, truncation, judge, and successful classifications.

Confidence Score: 5/5

The PR appears safe to merge because no eligible or previously reported blocking failure remains.

No blocking failure remains.

Important Files Changed

Filename	Overview
nemo_retriever/src/nemo_retriever/tools/evaluation/scoring.py	Adjusts failure-mode precedence and normalizes DataFrame generation-error values before classification.
nemo_retriever/tests/test_live_rag.py	Adds parametrized pipeline coverage for originating-stage failure classification.

_{Reviews (2): Last reviewed commit: "Merge branch 'main' into nvbug_6622224" | Re-trigger Greptile}