.NET: Merge AgentRunOptions.AdditionalProperties into ChatOptions.AdditionalProperties

[westey-m]

Motivation and Context

We should pass AgentRunOptions.AdditionalProperties to ChatOptions.AdditionalProperties in ChatClientAgent so that additional properties are replicated all the way down the stack.

The allows us to set a property on AgentRunOptions.AdditionalProperties without needing to break out of the abstraction and then access the property in a function tool via FunctionInvokingChatClient.CurrentContext.Options?.AdditionalProperties

#3179

Description

• Replicate AgentRunOptions.AdditionalProperties to ChatOptions.AdditionalProperties in ChatClientAgent
• Move unit tests for ChatOptions Merging in a separate class to reduce the size of ChatClientAgentTests
• Modify ChatOptionsMergingPrioritizesRequestOptionsOverAgentOptionsAsync to verify the new scenario

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
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This pull request implements the merging of AgentRunOptions.AdditionalProperties into ChatOptions.AdditionalProperties to enable proper propagation of additional properties through the agent execution stack. The PR also refactors ChatOptions merging tests into a dedicated test file for better organization.

Changes:

• Added logic to merge AgentRunOptions.AdditionalProperties into ChatOptions.AdditionalProperties with highest precedence
• Refactored ChatOptions merging unit tests from ChatClientAgentTests.cs into a new dedicated file ChatClientAgent_ChatOptionsMergingTests.cs
• Enhanced the ChatOptionsMergingPrioritizesRequestOptionsOverAgentOptionsAsync test to verify three-level precedence (agent → request → run options)

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Added ApplyAgentRunOptionsOverrides logic to merge AgentRunOptions.AdditionalProperties into ChatOptions.AdditionalProperties; minor code style improvement in AdditionalProperties iteration
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgent_ChatOptionsMergingTests.cs	New dedicated test file containing all ChatOptions merging tests, including enhanced test for three-level precedence validation
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentTests.cs	Removed ChatOptions merging tests (moved to dedicated file)