Honor raw content representations in Responses system and developer messages

[Laurianti]

Why

OpenAIResponsesChatClient rebuilds a system or developer message from ChatMessage.Text, so a ResponseContentPart or ResponseItem set as RawRepresentation on its contents never reaches the request. User messages already send both as is, and OpenAIChatClient does the same for system and developer messages.

ResponseContentPart part = ResponseContentPart.CreateInputTextPart("cached instructions");
ChatMessage system = new(ChatRole.System, [new TextContent("cached instructions") { RawRepresentation = part }]);

// Before: a system message item rebuilt from "cached instructions", without `part`.
// Now: a system message item whose content is `part`.

This is how a caller sets a provider field that Microsoft.Extensions.AI does not model on a system prompt, such as the prompt cache breakpoint in #7679.

Change

When any content of a system or developer message carries a raw ResponseItem or ResponseContentPart, the message is converted as a user message is: raw items are yielded as they are, raw parts and text become the parts of the message item, and parts between yielded items are grouped into their own item to keep the order. Contents that are neither raw nor text are left out, as before.

A system or developer message without raw contents is converted exactly as before.

Fixes #7679

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved blocking issues were identified.

Review effort: Lite
Findings: None

What changed in this PR

Updates Responses message conversion to preserve raw ResponseItem and ResponseContentPart representations in system and developer messages.

Changes:

• Added raw-representation-aware conversion and ordering.
• Added tests for system/developer messages and mixed content.

File	Description
test/​Libraries/​Microsoft.Extensions.AI.OpenAI.Tests/​OpenAIConversionTests.cs	Verifies raw parts, items, and ordering.
src/​Libraries/​Microsoft.Extensions.AI.OpenAI/​OpenAIResponsesChatClient.cs	Preserves raw instruction content during conversion.

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[dotnet-comment-bot]

‼️ Found issues ‼️

Project	Coverage Type	Expected	Actual
Microsoft.Extensions.Diagnostics.Testing	Line	99	98.65 🔻
Microsoft.Extensions.Telemetry	Line	93	92.64 🔻
Microsoft.Extensions.AI.OpenAI	Line	75	68.59 🔻
Microsoft.Extensions.AI.OpenAI	Branch	75	58.22 🔻
Microsoft.Extensions.DataIngestion.MarkItDown	Line	75	4.46 🔻
Microsoft.Extensions.DataIngestion.MarkItDown	Branch	75	0 🔻
Microsoft.Extensions.Diagnostics.ResourceMonitoring	Line	99	96.03 🔻
Microsoft.Extensions.Diagnostics.ResourceMonitoring	Branch	99	92.76 🔻
Microsoft.Extensions.Diagnostics.ResourceMonitoring.Kubernetes	Line	99	97.73 🔻
Microsoft.Extensions.ServiceDiscovery.Dns	Line	75	69.93 🔻
Microsoft.Extensions.ServiceDiscovery.Abstractions	Line	75	42.11 🔻
Microsoft.Extensions.ServiceDiscovery.Abstractions	Branch	75	42.86 🔻
Microsoft.Extensions.ServiceDiscovery	Line	75	67.21 🔻
Microsoft.Extensions.ServiceDiscovery	Branch	75	71.43 🔻
Microsoft.Extensions.ServiceDiscovery.Yarp	Line	75	73.85 🔻
Microsoft.Extensions.ServiceDiscovery.Yarp	Branch	75	70 🔻
Microsoft.Extensions.VectorData.Abstractions	Line	75	37.39 🔻
Microsoft.Extensions.VectorData.Abstractions	Branch	75	22.73 🔻

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

Project	Expected	Actual
Microsoft.Extensions.Http.Diagnostics	94	95
Microsoft.Gen.BuildMetadata	97	100
Microsoft.Gen.MetadataExtractor	57	73
Microsoft.Gen.MetricsReports	67	69
Microsoft.Extensions.AI.Abstractions	82	86
Microsoft.Extensions.AI.Evaluation.NLP	0	78
Microsoft.Extensions.Caching.Hybrid	82	84
Microsoft.Extensions.DataIngestion	75	89
Microsoft.Extensions.DataIngestion.Markdig	75	90
Microsoft.Extensions.Http.Resilience	97	100

Full code coverage report: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1613832&view=codecoverage-tab