.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 depreciated 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.

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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