Fix obsm key in cellmapper_scvi

[rcannood]

Describe your changes

cellmapper_scvi crashes on every single run, right at the end -- after it has trained scVI/PoissonVI on a GPU.

CellMapper.map_obsm() writes its result to f"{key}{prediction_postfix}":

output_key = f"{key}{prediction_postfix}"
self.query.obsm[output_key] = query_data

So map_obsm(key="mod2", prediction_postfix="pred") produces obsm["mod2pred"], but the script reads obsm["mod2_pred"] -> KeyError. Its sibling cellmapper_linear already reads the right key; only the scvi variant drifted.

Worth flagging that this was invisible in the last run: labels_tw.config sets errorStrategy = 'ignore' after three attempts, so a method that reliably crashes shows up as missing rows in scores.yaml rather than as a failure. Might be worth a separate look at how we surface "crashed" vs "no score".

Part of a series of PRs coming out of a pre-run review of the benchmark.

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