.NET: Add Magentic Orchestration Sample

[Copilot]

Motivation and Context

Ports the Python Magentic multi-agent orchestration sample to .NET, giving .NET developers a ready-to-run example of the MagenticWorkflowBuilder pattern for coordinating specialized agents on complex analytical tasks.

Description

New Magentic Orchestration Sample

Adds dotnet/samples/03-workflows/Orchestration/Magentic/ — a .NET port of the Python magentic.py workflow sample. The sample builds a MagenticWorkflowBuilder orchestration with three roles:

• ResearcherAgent – gathers factual background information
• CoderAgent – uses HostedCodeInterpreterTool for quantitative analysis
• MagenticManager – plans the work, tracks progress via a ledger, and decides who acts next

Key aspects demonstrated:

• Building a Magentic workflow with MagenticWorkflowBuilder (max rounds, stalls, resets)
• Combining standard responses-based agents with a code-interpreter-enabled participant
• Streaming orchestration events: initial plan, replans, and progress-ledger updates
• Printing the final multi-agent conversation transcript
• Graceful PauseIfInteractive that skips when stdin or stdout is redirected (CI-friendly)

Closes #5771

Other Changes

• Registered the new Magentic.csproj in the solution file
• Added the sample entry to the 03-workflows/README.md index

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.