.NET: Update Microsoft.Extensions.AI to 11.0.0 - #10349

Motivation & Context

Agent Framework currently pins Microsoft.Extensions.AI packages to 10.11.0. Updating to 11.0.0 allows applications to receive upstream fixes through the framework dependency graph, including the completed streaming image result from HostedImageGenerationTool.

Description & Review Guide

• What are the major changes? Updates Microsoft.Extensions.AI, Microsoft.Extensions.AI.Abstractions, and Microsoft.Extensions.AI.OpenAI to 11.0.0. Aligns directly required runtime dependencies by target framework and updates two samples with explicit package versions. Adds regression coverage for completed streaming image results when partial image streaming is not configured.
• What is the impact of these changes? Consumers receive MEAI 11 fixes without application-level overrides. OpenAI remains at 2.14.0 because MEAI.OpenAI 11.0.0 is not runtime-compatible with the experimental streaming API changes in OpenAI 2.15.0. No Agent Framework public API changes are introduced.
• What do you want reviewers to focus on? Please review the target-specific dependency versions and the streaming image regression scenario.

Related Issue

Fixes #10296

AI Assistance

• No material AI assistance was used.
• This is an AI-assisted contribution. I reviewed, understood, and verified all submitted content and accept responsibility for it.

AI assistance details: Copilot assisted with dependency analysis, implementation, regression coverage, validation, and this pull request description.

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 links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• 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.