Fix(lesson 04): Replaced deprecated AgentThread with AgentSession for context management

[marietta-a]

Description

Fixes CS1061 and CS0246 compilation errors in 04-dotnet-agent-framework.cs by updating to the correct agent API methods and types.

Addresses: #587

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Root Cause

During the creation of an agent with outdated methods, the code generated:

error CS1061: 'IChatClient' does not contain a definition for 'CreateAIAgent' ...
error CS0246: The type or namespace name 'AgentThread' could not be found ...
error CS1061: 'AIAgent' does not contain a definition for 'GetNewThread' ...

• The extension method .CreateAIAgent(...) is not defined for IChatClient. The supported method is .AsAIAgent(...).
• AgentThread is not part of the current framework; the correct type is AgentSession.
• GetNewThread() has been replaced by CreateSessionAsync() for session management.

Reference: Multi-turn Conversations, C#

Changes

• Replaced .CreateAIAgent(...) with .AsAIAgent(...)
• Replaced AgentThread with AgentSession
• Replaced .GetNewThread() with .CreateSessionAsync()
• Updated inline comments to reflect the correct agent/session workflow

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Testing

Verified code compiles and runs without errors on:

• OS: Windows 11, macOS Tahoe 26.2
• .NET SDK: 10.0.301
• Microsoft.Extensions.AI@10.*
• Microsoft.Extensions.AI.OpenAI@10.-
• Microsoft.Agents.AI.OpenAI@1.-

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This PR ensures the lesson‑04 sample aligns with the current APIs and demonstrates proper multi-turn conversation handling.

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