.NET: Fix stop sequence array serialization

[VectorPeak]

Motivation & Context

This PR fixes Chat Completions request serialization for multiple stop sequences by using the JSON metadata that matches the value being written.

Chat Completions requests support stop as either a single string or an array of strings. StopSequencesConverter already deserializes array values with string-list metadata, but the corresponding write path used message-content-part metadata when serializing StopSequences.Sequences.

The current write path was built with metadata for message content parts:

ChatCompletionsJsonContext.Default.IReadOnlyListMessageContentPart

That metadata describes message content values, not the IList<string> stored in StopSequences.Sequences. For requests that include multiple stop sequences, the converter should emit a JSON string array such as:

"stop": ["END", "STOP"]

Using the string-list metadata keeps the serializer consistent with the existing read path and with the expected Chat Completions request shape, while leaving single stop sequences and null stop values unchanged.

Description & Review Guide

What are the major changes?

• Serialize StopSequences.Sequences with the existing string-list JSON type information:

ChatCompletionsJsonContext.Default.IListString

• Add a focused serialization regression test for a Chat Completions request with multiple stop sequences.

What is the impact of these changes?

• Multiple stop sequences are emitted as a JSON string array, matching the existing deserialization path and the expected Chat Completions request shape.
• The affected path is CreateChatCompletion.Stop -> StopSequencesConverter.Write() -> StopSequences.Sequences -> the JSON stop field.
• Single stop sequences and null stop values are unchanged.

What do you want reviewers to focus on?

• Whether StopSequencesConverter should use the same string-list metadata for reading and writing stop sequence arrays.
• Whether the regression test captures the multiple-stop-sequence request serialization path clearly enough.

Related Issue

No linked issue. This is a small deterministic bug fix found while reviewing the Chat Completions request serialization path. I searched open issues and PRs for matching StopSequences / stop sequence serialization reports and did not find an existing one.

Contribution Checklist

• The code builds clean without any errors or warnings. Local build was not run because dotnet is not available in this environment.
• All unit tests pass, and I have added new tests where possible. Added a focused unit test; local test execution was blocked because dotnet is not available in this environment.
• 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

Fixes .NET OpenAI ChatCompletions “stop” serialization so that multiple stop sequences are emitted as a JSON string array (matching the expected wire format) and adds a unit test to guard the behavior.

Changes:

• Update StopSequencesConverter.Write to serialize Sequences using the source-generated IList<string> metadata instead of an unrelated type info.
• Add a serialization unit test asserting "stop" is written as a JSON array when multiple stop sequences are provided.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/OpenAIChatCompletionsSerializationTests.cs	Adds a regression test validating "stop" is serialized as a string array for multiple stop sequences.
dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/ChatCompletions/Models/StopSequences.cs	Fixes the converter’s serialization path to use the correct source-generated type info for IList<string>.

[VectorPeak]

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