Add AssistantReasoningEvent and AssistantReasoningDeltaEvent support to GitHubCopilotAgent

[Copilot]

Motivation and Context

GitHub Copilot SDK emits AssistantReasoningEvent and AssistantReasoningDeltaEvent for model reasoning/thinking output. GitHubCopilotAgent was not handling these events, causing reasoning content to be lost or stored as generic AIContent instead of the semantically correct TextReasoningContent.

Description

C# Implementation (GitHubCopilotAgent.cs)

• Added case handlers for AssistantReasoningDeltaEvent and AssistantReasoningEvent
• Map AssistantReasoningDeltaEvent.Data.DeltaContent → TextReasoningContent (streaming)
• Map AssistantReasoningEvent.Data.Content → TextReasoningContent (complete)
• Handle null data/content gracefully

Python Implementation (_agent.py)

• Added handlers for SessionEventType.ASSISTANT_REASONING_DELTA and SessionEventType.ASSISTANT_REASONING
• Create AgentResponseUpdate with Content.from_text_reasoning()

Tests

• C#: 6 unit tests covering event conversion and null handling
• Python: 2 streaming tests with reasoning event fixtures

Follows existing event handling patterns (e.g., AssistantMessageDeltaEvent → TextContent).

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.

Original prompt

Teach the GitHubCopilotAgent about AssistentReasoningEvent and AssistantReasoningDeltaEvent, mapping them to TextReasoningContent.

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