Add breaking change documentation for FromKeyedServicesAttribute.Key nullability

[Copilot]

Fixes #486863

This PR adds documentation for the breaking change in .NET 8 where FromKeyedServicesAttribute.Key changed from a non-nullable object to a nullable object? to support unkeyed services and inheritance scenarios.

Changes Made

• Created breaking change document: docs/core/compatibility/extensions/8.0/fromkeyedservicesattribute-key-nullable.md

  • Documents the property type change from object to object?
  • Explains the reasoning: support for keyed services to annotate parameters as unkeyed
  • Provides code examples showing how to handle the nullable Key property
  • Categorized as "Source incompatible" breaking change

• Updated navigation: Added entries to both the table of contents (toc.yml) and the .NET 8 index page (8.0.md) under the Extensions section

Note on API Name

The original issue referenced FromServiceKeyAttribute.Key, but the correct API name is FromKeyedServicesAttribute.Key. This has been corrected in the documentation based on the official Microsoft Learn API documentation.

The breaking change allows developers to explicitly indicate when a parameter should be resolved without a key, which is particularly useful when both keyed and unkeyed services are registered for the same type.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Breaking change]: FromKeyedServicesAttribute.Key may be null</issue_title>
<issue_description>### Description

FromServiceKeyAttribute has been extended to support null (for unkeyed) and inheritance.

Version

.NET 8

Previous behavior

FromServiceKeyAttribute.Key was a non-nullable object object.

New behavior

FromServiceKeyAttribute.Key is a nullable object object?.

Type of breaking change

• Binary incompatible: Existing binaries might encounter a breaking change in behavior, such as failure to load or execute, and if so, require recompilation.
• Source incompatible: When recompiled using the new SDK or component or to target the new runtime, existing source code might require source changes to compile successfully.
• Behavioral change: Existing binaries might behave differently at run time.

Reason for change

See dotnet/runtime#113585. Support was added for keyed services to annotate parameters as unkeyed.

Recommended action

Adjust any caller that is using FromServiceKeyAttribute to accept a null value for Key.

Feature area

Extensions

Affected APIs

Microsoft.Extensions.DependencyInjection.FromServiceKeyAttribute.Key

---

Associated WorkItem - 486863</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #46269

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Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/8.0.md	Breaking changes in .NET 8
docs/core/compatibility/extensions/8.0/fromkeyedservicesattribute-key-nullable.md	FromKeyedServicesAttribute.Key can be null
docs/core/compatibility/toc.yml	docs/core/compatibility/toc