.NET: test: add AgentResponse edge case coverage

[challaravinath]

Motivation and Context

AgentResponse had no tests covering null ChatResponse construction,
Text property behavior with non-text content, Text after Messages
is set to null, or message order preservation in ToAgentResponseUpdates.
These are gaps in critical response handling paths.
-->

Description

• Add test verifying null ChatResponse constructor throws ArgumentNullException
• Add test verifying Text returns empty string when all content is non-text
• Add test verifying Text returns empty string after Messages is set to null
• Add test verifying ToAgentResponseUpdates preserves message order across multiple messages

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• No breaking change test only, no production code modified

[Copilot]

Pull request overview

Adds targeted unit tests in the .NET Abstractions test suite to cover previously untested AgentResponse edge cases, improving confidence in response handling behavior without modifying production code.

Changes:

• Added a constructor guard test for AgentResponse(ChatResponse) when passed null.
• Added Text behavior tests for non-text-only content and for Messages being set to null.
• Added a test ensuring ToAgentResponseUpdates() preserves message ordering across multiple messages.