Fix concurrent projector cache test to avoid DB queries in thread loop

Move database query outside thread loop to properly test concurrent access
to the same document instance. This eliminates 1000 DB queries, reduces
connection pool contention, and makes the test faster and more reliable.

Addresses review comment: mongodb#6112 (comment)

Author: skcc321

spec/mongoid/fields/performance_spec.rb

@@ -443,12 +443,12 @@
 
       it 'handles concurrent projector cache access safely' do
         band = Band.create!(name: 'Test')
+        limited = Band.only(:name).find(band.id)
         errors = Concurrent::Array.new
 
         threads = Array.new(10) do
           Thread.new do
             100.times do
-              limited = Band.only(:name).find(band.id)
               limited.attribute_missing?('rating')
             rescue StandardError => e
               errors << e