.NET: Support reasoning events in AGUI

[jeffinsibycoremont]

Motivation and Context

Relates to the .NET support for reasoning mentioned in #2619 and parent issue #2558.

Description

7 new events based on agui spec https://docs.ag-ui.com/concepts/reasoning (note 'thinking' is decpriated and the convention now is 'reasoning' https://docs.ag-ui.com/concepts/reasoning#deprecated-events)

• ReasoningStartEvent
• ReasoningMessageStartEvent
• ReasoningMessageContentEvent
• ReasoningMessageEndEvent
• ReasoningEndEvent
• ReasoningMessageChunkEvent
• ReasoningEncryptedValueEvent

Outbound path (AsAGUIEventStreamAsync)

TextReasoningContent from the IChatClient pipeline is now converted into the explicit AG-UI reasoning lifecycle

(REASONING_START → REASONING_MESSAGE_START → REASONING_MESSAGE_CONTENT → REASONING_MESSAGE_END → REASONING_END).

ProtectedData is emitted as REASONING_ENCRYPTED_VALUE inline after the content delta. Content with only ProtectedData (no visible text) is not dropped - the block is opened and the encrypted value emitted without a content delta.

Inbound path (AsChatResponseUpdatesAsync)

A new ReasoningMessageBuilder (following the TextMessageBuilder pattern) handles both the explicit lifecycle form and the chunk shorthand form (REASONING_MESSAGE_CHUNK).

ReasoningStartEvent/ReasoningEndEvent are consumed as no-op bracket markers. The builder enforces the same invariants as TextMessageBuilder - overlapping starts and mismatched ends throw InvalidOperationException.

Request payload path (AsChatMessages)

A new AGUIReasoningMessage handles role: "reasoning" messages in the frontend's POST payload. Without this, multi-turn conversations with reasoning enabled would fail with 400 Bad Request on the second request, because AGUIMessageJsonConverter had no case for the "reasoning" role discriminator. The message is converted to TextReasoningContent preserving both visible text and the encrypted thinking token (ProtectedData).

Path	Direction	Method	Converts
Events (SSE streaming)	Server → Client (produce)	AsAGUIEventStreamAsync	TextReasoningContent → REASONING_* events
Events (SSE streaming)	Client → Server (consume)	AsChatResponseUpdatesAsync	REASONING_* events → TextReasoningContent
Messages (POST body)	Server ← Client (receive)	AsChatMessages	AGUIReasoningMessage → TextReasoningContent
Messages (POST body)	Client → Server (send)	AsAGUIMessages	TextReasoningContent → AGUIReasoningMessage

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[jeffinsibycoremont]

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[jeffinsibycoremont]

MEAI uses a MessageId in ChatResponseUpdate. ChatResponseUpdates contents can include TextReasoningContent. The MessageId emitted for reasoning events should be a 'unique identifier for the reasoning message' as per https://docs.ag-ui.com/concepts/reasoning and others such as the typescript ag-ui sdk depend on the IDs being unique https://github.com/ag-ui-protocol/ag-ui/blob/bd5f93f29b21ad150d671acb3e22db4c5b1dbd74/sdks/typescript/packages/client/src/apply/default.ts#L920.

As such 01a8291 creates a new GUID for reasoning content.

Similarly, despite ReasoningStart and ReasoningEnd events being no-ops, the protocol suggests the mesage ID of these should be unique as well.

Note that the current implementation in this PR means there is no messageId link between a reasoning message and text message as the protocol does not explicitly specify any such relationship e.g. ReasoningMessage has a unique GUID instead of being reasoning-${chatResponse.MessageId}.

This image from the docs suggests that RasoningStart/RasoningEnd be traced back to the related ReasoningMessage via the messageId, but Im wary of making such a link as there is nothing explicitly defined and could lead to a breaking change if we were to change this in the future.

Happy to update if I have misunderstood :)