[#13454] Refactor instructor search page to remove unnecessary API calls

[codigm]

Fixes #13454

Outline of Solution

loadInstructorPrivilege was being called for each unique student section
instead of once per course, causing redundant API calls. Results also weren't
cached across searches.

Changes:

• getPrivileges() now fetches once per unique courseId using forkJoin with
  a keyed Record instead of an unkeyed array
• Added a privilegeCache Map to avoid re-fetching on subsequent searches
• combinePrivileges() uses Map lookup by courseId instead of relying on
  forkJoin array ordering via shift()
• Removed the TransformedInstructorSearchResult wrapper interface
• Simplified the search() RxJS pipeline
• Updated tests and added a caching test

[github-actions]

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