Replace includes with eager_load for optimal single-query performance

- Replace all includes() with eager_load() in find, find_by, and where methods
- eager_load forces single LEFT OUTER JOIN queries instead of potential N+1 queries
- Update test expectations to validate single query performance
- All query methods now use optimized single queries:
  * Car.find(id) - 1 query with full JOIN
  * Car.find_by(fuel_type: 'hybrid') - 1 query with full JOIN
  * Car.where(fuel_type: 'hybrid') - 1 query with full JOIN
  * Chained where() queries - 1 query with full JOIN

Performance improvements:
- Eliminated N+1 queries for find and find_by methods
- Consistent single-query behavior across all query methods
- Proper association loading with eager_load vs includes
- All 58 tests passing with optimal performance

spec/active_record/aux_table_spec.rb

@@ -496,8 +496,8 @@ RSpec.describe ActiveRecord::AuxTable do
       end
 
       describe "query count validation" do
-        # TODO: These tests currently document the performance issues with auxiliary tables
-        # The auto-join functionality needs to be properly implemented to avoid N+1 queries
+        # These tests validate the performance optimizations using eager_load
+        # All query methods now use single queries with proper LEFT OUTER JOINs
 
         it "loads single model with auxiliary data in one query using find" do
           query_count =
@@ -509,7 +509,7 @@ RSpec.describe ActiveRecord::AuxTable do
             end
 
           # TODO: Should be 1 query, but currently 2 due to auxiliary record loading
-          expect(query_count).to eq(2)
+          expect(query_count).to eq(1)
         end
 
         it "loads single model with auxiliary data in one query using find_by" do
@@ -521,8 +521,8 @@ RSpec.describe ActiveRecord::AuxTable do
               car.engine_size
             end
 
-          # TODO: Should be 1 query, but currently 2 due to auxiliary record loading
-          expect(query_count).to eq(2)
+          # Excellent! Now using eager_load for single query performance
+          expect(query_count).to eq(1)
         end
 
         it "loads multiple models with auxiliary data in one query using where" do