Refactor: Address follow-up suggestions from !213378 and !213178

What does this MR do and why?

This MR addresses follow-up suggestions from !213378 (merged) and !213178 (merged) to improve code readability, test quality, and separation of concerns.

Changes:

1. Improve XOR validation readability in VulnerabilitiesPerSeverityResolver

   • Replace ^ operator with explicit boolean logic for better clarity
   • Suggested by @charlieeekroon in !213178 (merged)#note_2898652959

2. Remove redundant "Optional" text from argument descriptions

   • The required: false already indicates optionality
   • Suggested by @charlieeekroon in !213178 (merged)#note_2898621449

3. Add :aggregate_failures to test cases for better test output

   • Suggested by @bala.kumar in !213178 (merged)

4. Move date mapping logic from finder to resolver (addressing Schmil's suggestion)

   • Resolver now passes bucket_start_date and bucket_end_date directly
   • Finder no longer manipulates search_params with .except()
   • Keeps finder focused on data retrieval, not business logic
   • Suggested by @schmil.monderer in !213378 (merged)#note_2901630142

Related issues

Closes #581319 (closed)

Related merge requests

• !213378 (merged) - Remove 1 year time bound limitation on vulnerability over time chart
• !213178 (merged) - fix: remove max range limit in VulnerabilitiesPerSeverityResolver