Add AsChatClient for OpenAI's AssistantClient

[stephentoub]

• Adds ChatOptions/ChatCompletion/StreamingChatCompletionUpdate.ChatThreadId.
• Adds an AsChatClient extension method for creating an IChatClient from an AssistantClient.
• Updates FunctionInvokingChatClient to have KeepFunctionCallingMessages
  be false by default, and to handle ChatThreadId.
• Fixes handling of ChatCompletion.Usage in ToChatCompletion{Async}.

[dotnet-comment-bot]

‼️ Found issues ‼️

Project	Coverage Type	Expected	Actual
Microsoft.Extensions.Caching.Hybrid	Line	86	82.77 🔻
Microsoft.Extensions.AI.OpenAI	Line	77	71.76 🔻
Microsoft.Extensions.AI.OpenAI	Branch	77	53.47 🔻
Microsoft.Gen.MetadataExtractor	Line	98	57.35 🔻
Microsoft.Gen.MetadataExtractor	Branch	98	62.5 🔻
Microsoft.Extensions.AI.Ollama	Line	80	78.25 🔻

🎉 Good job! The coverage increased 🎉
Update MinCodeCoverage in the project files.

Project	Expected	Actual
Microsoft.Extensions.AI	88	89
Microsoft.Extensions.AI.Abstractions	83	84

Full code coverage report: https://dev.azure.com/dnceng-public/public/_build/results?buildId=943532&view=codecoverage-tab

[dotnet-comment-bot]

‼️ Found issues ‼️

Project	Coverage Type	Expected	Actual
Microsoft.Extensions.Caching.Hybrid	Line	86	82.92 🔻
Microsoft.Extensions.AI.Ollama	Line	80	78.11 🔻
Microsoft.Extensions.AI.OpenAI	Line	77	71.92 🔻
Microsoft.Extensions.AI.OpenAI	Branch	77	53.71 🔻
Microsoft.Gen.MetadataExtractor	Line	98	57.35 🔻
Microsoft.Gen.MetadataExtractor	Branch	98	62.5 🔻

🎉 Good job! The coverage increased 🎉
Update MinCodeCoverage in the project files.

Project	Expected	Actual
Microsoft.Extensions.AI.AzureAIInference	91	92
Microsoft.Extensions.AI.Abstractions	83	85

Full code coverage report: https://dev.azure.com/dnceng-public/public/_build/results?buildId=944576&view=codecoverage-tab

[Copilot]

PR Overview

This pull request adds chat thread IDs for managing stateful conversations, introduces a new AsChatClient extension method for AssistantClient, and modifies the default value for KeepFunctionCallingMessages in FunctionInvokingChatClient from true to false. It also refactors how function-calling messages are retained or cleared, updates the handling of usage data aggregation, and includes various serialization, testing, and maintenance improvements.

• Adds ChatThreadId properties to ChatOptions, ChatCompletion, and StreamingChatCompletionUpdate.
• Changes the default KeepFunctionCallingMessages to false and removes the message-tracking cleanup code.
• Provides a new AsChatClient(AssistantClient, string, string?) extension method returning OpenAIAssistantClient.
• Enhances usage content coalescing in streaming updates and aggregates usage details properly in ChatCompletion.

Changes

File	Description
src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIAssistantClient.cs	Implements an IChatClient that uses an AssistantClient, optionally with a shared thread ID.
src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs	Updates default KeepFunctionCallingMessages, refactors function call message handling and usage updates.
src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatCompletion.cs	Adds ChatThreadId property and usage aggregation improvements.
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/ChatCompletion/ChatOptionsTests.cs	Tests for new ChatThreadId property.
src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIClientExtensions.cs	Implements the new AsChatClient method for AssistantClient.
src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatOptions.cs	Adds ChatThreadId property to ChatOptions and updates clone logic.
src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIChatClient.cs	Minor internal refactoring for default endpoint handling.
test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientTests.cs	Updates tests to cover the new KeepFunctionCallingMessages default and thread ID behaviors.
src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIModelMapper.ChatCompletion.cs	Exposes DefaultParameterSchema as a property and updates usage accordingly.
src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIJsonContext.cs	Adds support for string array serialization in JSON context.
src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/StreamingChatCompletionUpdate.cs	Adds ChatThreadId property for streaming completion updates.
src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatMessage.cs	Adds a shallow Clone method for ChatMessage.
src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/StreamingChatCompletionUpdateExtensions.cs	Improves how usage content is extracted and aggregated in streaming updates.

Copilot reviewed 14 out of 14 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs:283

• Fix the typo 'Expecteded' to 'Expected'.

Debug.Assert(functionCount > 0, "Expecteded {nameof(functionCount)} to be > 0, got {functionCount}.");

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[SteveSandersonMS]

Looks good!

• Personally I'd be more inclined to leave KeepFunctionCallingMessages on by default as per https://github.com/dotnet/extensions/pull/5852/files?w=1#r1948821474 but I know this is a subjective call
• Should there be some additional tests covering the FunctionInvokingChatClient behaviors around its detection/handling of ChatThreadId?
• Technically also we have a load of integration tests for OpenAIChatClient that don't have an equivalent in the new OpenAIAssistantClient. However I know it's a huge pain to create them so will leave it with you to decide if there's enough value in doing so.

[stephentoub]

Should there be some additional tests covering the FunctionInvokingChatClient behaviors around its detection/handling of ChatThreadId?

Yes, added. Thanks.

[dotnet-comment-bot]

‼️ Found issues ‼️

Project	Coverage Type	Expected	Actual
Microsoft.Gen.MetadataExtractor	Line	98	57.35 🔻
Microsoft.Gen.MetadataExtractor	Branch	98	62.5 🔻
Microsoft.Extensions.Caching.Hybrid	Line	86	82.77 🔻
Microsoft.Extensions.AI.OpenAI	Line	77	71.71 🔻
Microsoft.Extensions.AI.OpenAI	Branch	77	53.28 🔻
Microsoft.Extensions.AI.Ollama	Line	80	78.2 🔻

🎉 Good job! The coverage increased 🎉
Update MinCodeCoverage in the project files.

Project	Expected	Actual
Microsoft.Extensions.AI.AzureAIInference	91	92
Microsoft.Extensions.AI.Abstractions	83	85

Full code coverage report: https://dev.azure.com/dnceng-public/public/_build/results?buildId=946767&view=codecoverage-tab