Fix manifest reference path conversion

[fallintoplace]

Summary

Fix NeMo-to-Lhotse manifest conversion so reference recordings keep their own manifest path instead of reusing the target path.

Root cause

convert_manifest_nemo_to_lhotse() loaded item_reference_key, but the reference branch passed item_target_key into _as_relative(). That made reference-only manifests crash with an unbound local and could serialize the wrong relative metadata when target and reference paths differed.

What changed

• route the reference path override through item_reference_key
• add a regression test for manifests that have reference_filepath without target_filepath
• add a regression test for manifests whose target and reference paths are both present and intentionally different

Impact

Reference-only manifests now convert successfully, and mixed target/reference manifests preserve the correct relative path metadata for both recordings.

Validation

• uv run pytest tests/collections/audio/test_audio_datasets.py -k "convert_manifest_nemo_to_lhotse_with_reference_only or convert_manifest_nemo_to_lhotse_with_different_target_and_reference_paths" -v
• uv run pytest tests/collections/audio/test_audio_datasets.py -k "audio_to_target_with_reference_dataset" -v
• uv run --with black==24.10.0 black --check nemo/collections/audio/data/audio_to_audio_lhotse.py tests/collections/audio/test_audio_datasets.py
• uv run isort --check-only nemo/collections/audio/data/audio_to_audio_lhotse.py tests/collections/audio/test_audio_datasets.py

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