[release/10.0] Fix InvalidCastException in OriginalValues.ToObject() with nested nullable complex properties

[Copilot]

Fixes #37516

Description

When calling context.Entry(...).OriginalValues.ToObject(), an InvalidCastException is thrown when entities have nested nullable complex properties and any of those nested properties are null.

Customer impact

This prevents customers from using OriginalValues.ToObject() with nested nullable complex properties.

Workaround: Customers can set the AppContext switch Microsoft.EntityFrameworkCore.Issue37516 to true to revert to the old behavior, but this disables the nullable complex property feature added in 10.0.2.

How found

Customer reported on 10.0.2.

Regression

Yes, this is a regression from 10.0.1, introduced in #37302

Testing

Tests added.

Risk

Low. Quirk mode added.

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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 with the requested template format. The issue number placeholder (#XXXXX) should be replaced with the actual issue number when available.

[AndriySvyryd]

@copilot Unskip this test and improve the fix to handle this scenario as well.
Try to explore other related scenarios and add more tests

[Copilot]

Unskipped and fixed the test. The fix includes:

1. Recursive null complex type tracking (IdentifyNullComplexTypes)
2. Property chain navigation for getting values from nested complex types
3. Only setting complex properties declared directly on the current structural type to avoid navigation issues

All 4 tests pass including the complex collection test with double nested nullable complex properties. Commit a2c2438