Improve Assert.AreSame message for nulls

[Evangelink]

Fix #7421

[Copilot]

Pull request overview

This PR addresses #7421 by improving MSTest Assert.AreSame/Assert.AreNotSame failure messages when null values are involved, making failures easier to understand without needing to infer nullness from the inputs.

Changes:

• Add localized resource strings for null-specific AreSame/AreNotSame failure cases.
• Update Assert.AreSame/Assert.AreNotSame failure construction to include null-specific wording.
• Add unit tests asserting the updated message output across string and interpolated-string overloads.

Show a summary per file

File	Description
test/UnitTests/TestFramework.UnitTests/Assertions/AssertTests.AreSame.cs	Adds test coverage for the new null-specific failure messages for AreSame and AreNotSame.
src/TestFramework/TestFramework/Assertions/Assert.AreSame.cs	Implements null-specific message formatting for AreSame (expected/actual null) and AreNotSame (both null), including interpolated-string handler paths.
src/TestFramework/TestFramework/Resources/FrameworkMessages.resx	Adds new resource entries used to format the null-specific assertion failure messages.
src/TestFramework/TestFramework/Resources/xlf/FrameworkMessages.*.xlf	Adds corresponding localization entries for the newly introduced resource keys across supported cultures.

Copilot's findings

• Files reviewed: 16/16 changed files
• Comments generated: 0