Update to OpenAI 2.6.0

[stephentoub]

In addition to updating the dependency, it now enables us to do additional things:

• Propagate ChatOptions/EmbeddingGeneratorOptions.ModelID to allow overriding the model per-request.
• Propagate AIContent results of tool invocations using non-string tool results.
• Add support for container file annotations.
• Fixed handling of GetResponse{Streaming}Async to use RequestOptions
• Remove hacky serialization code around ResponseCreationOptions.

A previous PR added HostedFileContent.Name/MediaType, but we missed adding HasTopLevelMediaType (which both DataContent and UriContent have). I had a need for that here, so included it.

Also added some more unit tests to improve code coverage for the response chat client.

And also fixes #6997.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull Request Overview

This pull request adds comprehensive support for function result content types in the OpenAI Responses API client and enables per-request model ID overrides across all OpenAI clients. The implementation includes extensive test coverage for various content types in tool call results.

Key changes:

• Added support for AIContent types (TextContent, DataContent, UriContent, HostedFileContent) as function results in the Responses API
• Implemented per-request ModelId override capability in ChatOptions for Chat Completion, Responses, and Embedding clients
• Updated to OpenAI SDK 2.6.0 and added HasTopLevelMediaType method to HostedFileContent

Reviewed Changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
OpenAIResponsesChatClient.cs	Adds serialization logic for AIContent types in tool call results, implements model override via JsonPatch, and reorders annotation handling
OpenAIClientExtensions.cs	Adds PatchModelIfNotSet helper method for setting model in JsonPatch when not already set
OpenAIChatClient.cs	Adds model override support using PatchModelIfNotSet
OpenAIEmbeddingGenerator.cs	Adds model override support using PatchModelIfNotSet
OpenAIJsonContext.cs	Adds JSON serialization support for FunctionToolCallOutputElement list
HostedFileContent.cs	Adds HasTopLevelMediaType method to check media type prefix
OpenAIResponseClientTests.cs	Adds 33 new unit tests covering model overrides, tool result content types, and edge cases
OpenAIResponseClientIntegrationTests.cs	Adds 5 integration tests for tool call results with various content types
OpenAIChatClientTests.cs	Adds 2 unit tests for model override in streaming and non-streaming scenarios
Microsoft.Extensions.AI.OpenAI.csproj	Adds SCME0001 warning suppression and removes unused injector properties
General.props	Updates OpenAI package version to 2.6.0
CHANGELOG.md files	Documents new features and fixes
API baseline file	Adds new HasTopLevelMediaType API to manifest