.NET: Remove retired OpenAI Assistants integration tests

[Roger Barreto (rogerbarreto)]

Motivation & Context

OpenAI retired the Assistants API on August 26, 2026. The remaining integration test project now receives HTTP 404 from POST /v1/assistants, which causes the .NET integration job to fail even though the MAF Assistants extensions were removed previously.

Description & Review Guide

• What are the major changes? Remove the orphaned OpenAIAssistant.IntegrationTests project, its fixture and conformance tests, and its solution entry.
• What is the impact of these changes? CI no longer calls the retired Assistants endpoint. OpenAI Chat Completions and Responses integration coverage remains unchanged.
• What do you want reviewers to focus on? Confirm that no supported MAF behavior still depends on this retired API coverage.

Related Issue

N/A. The failure was identified in GitHub Actions run 33004224957.

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

Removes obsolete .NET integration tests for the retired OpenAI Assistants API while retaining Chat Completions and Responses coverage.

Changes:

• Deletes the Assistants fixture and conformance tests.
• Removes the retired test project from the solution.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
OpenAIAssistantStructuredOutputRunTests.cs	Removes structured-output tests.
OpenAIAssistantRunStreamingTests.cs	Removes streaming run tests.
OpenAIAssistantIRunTests.cs	Removes run tests.
OpenAIAssistantFixture.cs	Removes the retired API fixture.
OpenAIAssistantChatClientAgentRunTests.cs	Removes agent run tests.
OpenAIAssistantChatClientAgentRunStreamingTests.cs	Removes agent streaming tests.
OpenAIAssistant.IntegrationTests.csproj	Deletes the orphaned test project.
agent-framework-dotnet.slnx	Removes the project from the solution.

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[github-actions]

MAF Automated Review — Iteration 1

Result: No findings
Scope: full PR (1 commit(s)): e36dcc53c38e
Model: gpt-5.6-sol

Overview

This PR removes an orphaned integration-test project for the retired OpenAI Assistants endpoint and removes its solution entry. The supported Chat Completions and Responses integration projects remain registered, while both solution-based and filesystem-based CI discovery can no longer select the retired project. No production API or behavior changes were identified.

Reviewed the supplied pull-request change set across correctness, security/reliability, architecture, and failure behavior.
No publishable findings remained after source verification for this scope.