Fix iter_agent_groups for fastest scheduler

[Jonas-a-Zimmermann]

tackles #1210 by moving Base.map to Iterators.map and Ntuples.

Includes tests with schedulers fastest, Randomly(), ByID(), Partially(1), ByProperty(:weight), but any scheduler returning either a Vector or a KeySet should be fine.

Test Summary:   | Pass  Total     Time
Agents.jl Tests | 2514   2514  2m36.3s
     Testing Agents tests passed 

[Datseris]

I'm a bit confused about the usage of parenthesis here?

[Jonas-a-Zimmermann]

I believe these were added in a runic pass
could also have been my local formatter, but I ran runic over it afterwards
either way i did not add them manually

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.04%. Comparing base (8b5b456) to head (316974b).
⚠️ Report is 215 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1211       +/-   ##
===========================================
+ Coverage   70.12%   85.04%   +14.92%     
===========================================
  Files          42       42               
  Lines        2718     2936      +218     
===========================================
+ Hits         1906     2497      +591     
+ Misses        812      439      -373     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.