Improve usability of the Agency filter

[mxk0]

Summary

Improvements to consider:

• Consider information architecture (Jenn's note)
• Make it easier to find an agency (Michael's note)
• Users may expect the filters to only show options relevant to a result set. (For example, if you enter a search term, the filters should only show options relevant to the results for that term.)

Lots of discussion here.

Acceptance criteria

• Discussion had, follow-up tickets created

[chouinar]

A few other miscellaneous agency-related notes/thoughts:

• Some agencies are marked as test agencies during our transform process. This is based on a configuration taken from the legacy system for agencies that they filter out of certain parts of their system. In prod that list is GDIT,IVV,IVPDF,0001,FGLT,NGMS,NGMS-Sub1,SECSCAN - If you look at any opportunity assigned to these, you'll see they're clearly test opportunities, probably not something we want in our response.
• We'll probably need the frontend to fetch agencies in two parts. First via some GET /agencies endpoint that returns everything (after filtering out test agencies, maybe also zero-opportunity agencies) just to form a cache of values, then to use the facet counts from the search endpoint to display counts / only display anything that has at least one result in the full search results. If we wanted a slightly simpler approach for the frontend, we could technically try to build the hierarchy in the search response (the legacy system does that).

[mxk0]

We'll revisit this after the usability testing synthesis is complete. Short-term fixes will happen in #3057 and #3058.

[mxk0]

We addressed some of the bigger points of confusion already, ensuring the agency list is complete and removing the acronyms to decrease visual clutter. We are going to hold off on further work here until we have more data on usage of this filter.

[capriiis]

Crystabel will share Figma file of changes to be made before closing.