[release/10.0] Fix optional complex property default values tracking

[Copilot]

Fixes #37386
Port of #37387

Description
When optional complex properties transition from null to non-null with default-valued properties (e.g., entity.ComplexProp = new() { IntValue = 0 }), those default values were not being tracked or persisted to the database.

Customer impact
Applications using optional complex properties would lose data when setting a complex property from null to a new instance with default values. The properties with default values (like 0, false, default(DateTimeOffset)) would not be saved to the database, resulting in NULL values in the database columns instead of the expected default values.

Workaround: Users could explicitly mark the entity or properties as modified after setting the complex property, or set non-default values temporarily.

How found
2 customer reports on 10.0.x

Regression
No. Optional complex properties were introduced in EF 10.

Testing
Added a comprehensive test

Risk
Low - The fix is targeted and only affects optional nullable complex properties. The quirk mode switch provides an escape hatch for any unforeseen issues.

Original prompt

Port #37387, add a quirk mode switch named Issue37387

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[AndriySvyryd]

@copilot Update progress by rewriting the PR summary using this template:

Fixes #{number}

**Description**
{Issue description}

**Customer impact**
{How does the reported issue affects customer's app? Are there workarounds?}

**How found**
{Was it customer reported or found during verification? How many customers are affected?}

**Regression**
{Is it a regression from a released version? Which one?}

**Testing**
{How the changes were tested}

**Risk**
{Low/Medium/High, argument why}

[Copilot]

@copilot Update progress by rewriting the PR summary using this template:

Updated PR description using the servicing PR template. The description now includes all required sections: Description, Customer impact, How found, Regression, Testing, and Risk.

[Copilot]

Pull request overview

This PR ports the fix from PR #37387 and adds a quirk mode switch Microsoft.EntityFrameworkCore.Issue37387 to allow users to opt out of the behavior change. The fix addresses issue #37386 where default values in optional complex properties were not being tracked or saved when the complex property transitioned from null to non-null.

Changes:

• Added quirk mode switch to allow reverting to old behavior if needed
• Enhanced change detection to mark all inner properties as modified when optional complex properties transition from null to non-null
• Added comprehensive test coverage with provider-specific skips for known limitations
• Updated test infrastructure to support 4 nested test operations

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/EFCore/ChangeTracking/Internal/ChangeDetector.cs	Added quirk mode switch Issue37387 and implemented fix to detect and mark complex property changes from null to non-null, ensuring default values are persisted
test/EFCore.Specification.Tests/ComplexTypesTrackingTestBase.cs	Added test Can_save_default_values_in_optional_complex_property_with_multiple_properties with 3 nested operations, added test entities, and updated test infrastructure signature to support 4 nested operations
test/EFCore.InMemory.FunctionalTests/ComplexTypesTrackingInMemoryTest.cs	Updated test infrastructure signature and skipped new test due to InMemory provider limitation (issue #31464)
test/EFCore.SqlServer.FunctionalTests/ComplexTypesTrackingSqlServerTest.cs	Skipped new test for proxy configuration due to notification change tracking limitation (issue #36175)