Surface Responses API reasoning item content as TextReasoningContent

[LouisDeconinck]

Fixes #7714

OpenAIResponsesChatClient.ToChatMessages read reasoning text only from a ReasoningResponseItem's summary parts via GetSummaryText(). Responses API implementations that return raw chain-of-thought in the item's content field instead of summary (e.g. OpenRouter with gpt-oss-family models) produced a TextReasoningContent with empty Text, so the reasoning never surfaced to callers or OTel traces.

The OpenAI client doesn't expose content as a property (per openai/openai-dotnet#1329 the field exists in the schema but is never returned by the OpenAI service), but it is preserved in the item's JsonPatch. This change falls back to reading $.content from the patch when the summary is empty, concatenating the text of its reasoning_text parts — mirroring how #7295 reads reasoning_content via patch in OpenAIChatClient, and how the streaming branch surfaces response.reasoning_text.delta payloads.

No request-side change is needed: the reasoning item's RawRepresentation is preserved and round-trips the content field as-is.

Tests

• New ReasoningContent_NonStreaming test asserts a reasoning item carrying content (with an empty summary) produces a TextReasoningContent whose Text contains the concatenated reasoning. Verified the test fails before the fix (Text was empty).
• Microsoft.Extensions.AI.OpenAI.Tests: 390 passed / 0 failed on net8.0, net9.0, and net10.0.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

🔵 Needs a closer look

The fallback should only extract text from reasoning_text parts.

Pull request overview

Updates the OpenAI Responses API adapter to surface reasoning text stored in content when summary is empty.

Changes:

• Adds JSON-patch fallback extraction for reasoning content.
• Adds regression coverage for concatenated reasoning text.

File summaries

File	Description
test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIResponseClientTests.cs	Adds non-streaming reasoning-content regression coverage.
src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs	Implements the content fallback for reasoning text.

Review details

Suppressed comments (1)

src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs:971

• This fallback appends the text property from every object in content, rather than only from reasoning_text parts. If a response contains another content-part type with a text field, that text will be incorrectly exposed as TextReasoningContent; check the part's type before appending.

                    if (part.ValueKind == JsonValueKind.Object &&
                        part.TryGetProperty("text"u8, out JsonElement textElement) &&
                        textElement.ValueKind == JsonValueKind.String)
                    {

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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