Support AND semantics for multi-field collapsefield

Is your feature request related to a problem? Please describe. When collapsing on multiple fields (collapsefield=a,b), the semantics are OR / independent: a hit is dropped if it matches a higher-ranked hit on either field. There's no way to express the AND condition — i.e. drop a hit only when it matches a higher-ranked hit on all listed fields. This matters for result diversity / dedup cases where two documents should only be considered duplicates if they agree on every collapse field, not just one.

Describe the solution you'd like A way to choose AND semantics across multiple collapse fields, so a hit is collapsed only if it matches a higher-ranked hit on every field in the list (rather than any single one).

Describe alternatives you've considered Precomputing a single composite key field (concatenation of a + b) and collapsing on that. This works but requires a schema change and full reindex.

Additional context Docs for reference: https://docs.vespa.ai/en/querying/result-diversity#collapsefield