Add standard: search operator; remove file_type:

[bendichter]

Follow-up to #2882, which was pared down per review to just the performance fix. This PR carries the operator-set changes that were split out of it, and is stacked on that branch.

Re-Adding standard:

standard: was removed in 05fda3c pending a decision on the right metadata source. That decision is now made: it matches assetsSummary.dataStandard, the same version-level aggregation that #2882 pointed the other operators at. Real entries are names like "Neurodata Without Borders (NWB)" and "Brain Imaging Data Structure (BIDS)", so standard:nwb and standard:bids match by substring. Against production metadata, standard:nwb returns 137 dandisets and standard:bids returns 8.

Removing file_type:

Its remaining value is covered by standard:, and AssetsSummary has no encodingFormat or MIME field, so keeping it would have meant keeping the per-asset AssetSearch scan that #2882 moved the other operators off of. Dropping it takes AssetSearch out of this code path entirely, which in turn lets apply_search_filters drop its user parameter: the remaining filters only ever narrow the caller's queryset, and detail routes re-check embargo_status in get_object before returning anything.

There is no close-enough key for the parser's generic "did you mean" suggestion to fire on file_type (the best ratio is modified_after at 0.52), so it now gets an explicit 400 pointing at standard: and the file type filter in the search sidebar, with a test pinning that message.

This removes only the search operator. The faceted file_type query parameter on DandisetSearchQueryParameterSerializer is a separate feature and is untouched.

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