WE2.5.1 - article-country predictions available on the Search index

Implement articlecountry a new CirrusSearch keyword, still needs some review - https://phabricator.wikimedia.org/T386068

MLR Improvements

Deploy and test new MLR models https://phabricator.wikimedia.org/T385972
- A/B test reports are available at https://people.wikimedia.org/~gmodena/search/mlr/ab/2025-02/. I conducted a first round of analysis to identify potential regressions in the mlr2025-02 experiments compared to the control, as well as in the control compared to the mlr-2024 experiments. Model performance appears to be stable.
independentely analyzed results; will add further comments to T385972.
- Long comment here: https://phabricator.wikimedia.org/T385972#10589233
- TL;DR: Mostly as expected, though the Japanese new model is clearly better—so much so that it is weird, and if the stats were reversed, I would not deploy it. Not sure how to automate that intuition.
Some more thought will be required to understand some of the the interleaved experiment results.