.NET: Allow agents to opt into concurrent tool invocation

[ump45nose]

Motivation & Context

AsAIAgent() always left its default FunctionInvokingChatClient in serial invocation mode, even when the model returned multiple independent tool calls. This adds an explicit, backward-compatible opt-in for concurrent execution while preserving the current serial default.

Description & Review Guide

• What are the major changes? Add ChatClientAgentOptions.AllowConcurrentInvocation, pass it to the default function-invocation middleware, preserve it when cloning options, and cover both paths with focused unit tests.
• What is the impact of these changes? Existing agents remain serial by default; callers may opt in when their tools are safe to execute concurrently.
• What do you want reviewers to focus on? The separation between model-side AllowMultipleToolCalls and runtime function-invocation concurrency.

Related Issue

Fixes#7640

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Pull request overview

Adds opt-in concurrent tool invocation for .NET chat-client agents while retaining serial execution by default.

Changes:

• Adds and clones AllowConcurrentInvocation.
• Configures default function-invocation middleware.
• Adds focused option and middleware tests.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
ChatClientExtensions.cs	Applies concurrency configuration to new middleware.
ChatClientAgentOptions.cs	Exposes and clones the new option.
ChatClientExtensionsTests.cs	Tests enabling concurrent invocation.
ChatClientAgentOptionsTests.cs	Tests defaults and cloning.

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[ump45nose]

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[ump45nose]

Roger Barreto (@rogerbarreto) — The CLA is complete, both review threads are resolved, and the branch is now rebased onto current main. The focused option/middleware suite passes all 15 tests on the final head. When you have time, I would appreciate a review.

[westey (westey-m)]

Thanks for the contribution ump45nose