Reference fields should show a mixed-value indicator ("-") when multiple objects with different references are selected

How to reproduce:

1. Open the attached "IN-148270_ObjectSearchFieldBug" project
2. Open “SampleScene” Scene
3. In the Hierarchy panel, select multiple “ReferenceTest” objects
4. Observe the exception in the Console

Actual result: Selecting multiple “ReferenceTest” objects with different “Prefab” values throws “NotImplementedException” in the Console
Expected result: The “Prefab” field shows the mixed-value dash ("-"), with no exception in the Console

Reproducible with: 6000.0.1f1, 6000.0.79f1, 6000.3.20f1, 6000.5.4f1, 6000.6.0b5, 6000.7.0a3

Reproducible on: Windows 11 Enterprise (25H2)
Not reproducible on: No other environments tested

Notes:

• When “ReferenceTest” objects are selected separately, everything works fine
• In 6000.0.79f1 the Inspector field also becomes black, when you select multiple “ReferenceTest” objects, until you click on it
• The “Prefab” field uses [SearchContext], which makes Unity draw it with the Search version of ObjectField that doesn't support the mixed-value ("-") state, so the exception is thrown
• Error Thrown: “NotImplementedException: The method or operation is not implemented.”