Fix NullReferenceException when accessing null complex properties in TPH with shared columns

[Copilot]

Port PR #37389 to main

• Update PropertyAccessorsFactory.cs to preserve null checks for nullable reference type complex properties in TPH scenarios
• Add test cases in ComplexTypesTrackingSqlServerTest.cs for TPH shared complex property column scenarios
• Add model entities for test cases (InheritedItemBase, InheritedItem1, InheritedItem2, SharedPrice)
• Fix the check to verify if property's declaring type is a complex type (not if property itself is complex)
• Skip setting properties in null complex types during AcceptChanges
• Add filter to First() queries to avoid FirstWithoutOrderByAndFilterWarning
• Only add null checks for nullable complex properties (not required ones) to avoid breaking compiled model tests
• Run code review and security checks

All changes complete. The fix now correctly handles null complex types in two critical places:

1. PropertyAccessorsFactory: Preserves null checks when accessing properties declared in nullable complex types (checks ComplexProperty.IsNullable)
2. InternalEntryBase.AcceptChanges: Skips setting store-generated values on properties inside null complex types

The fix is optimized to only add null checks where actually needed (nullable complex properties), keeping compiled models clean and efficient while protecting against NRE in TPH scenarios with shared columns.

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[AndriySvyryd]

@copilot Microsoft.EntityFrameworkCore.Scaffolding.CompiledModelSqlServerTest.ComplexTypes and other implementations of the ComplexTypes test are failing with

System.AggregateException : Differences found in 2 files (Difference found in PrincipalBaseEntityType.cs) (Difference found in PrincipalDerivedEntityType.cs)\r\n---- System.Exception : Difference found in PrincipalBaseEntityType.cs\r\n-------- Assert.Equal() Failure: Strings differ\r\n                                  ↓ (pos 76408)\r\nExpected: ···"edType instance) => instance.Details,\r\n  "···\r\nActual:   ···"edType instance) => (instance == null ? d"···\r\n                                  ↑ (pos 76408)\r\n---- System.Exception : Difference found in PrincipalDerivedEntityType.cs\r\n-------- Assert.Equal() Failure: Strings differ\r\n                                  ↓ (pos 9682)\r\nExpected: ···"edType instance) => instance.Details,\r\n  "···\r\nActual:   ···"edType instance) => (instance == null ? d"···\r\n                                  ↑ (pos 9682)

[Copilot]

Fixed in 4e16643. Added check for ComplexProperty.IsNullable so null checks are only generated for nullable complex properties, not required ones. This prevents unnecessary null checks in compiled models while still protecting against NRE for nullable complex properties in TPH scenarios.