.NET: Improve the getting started samples

[markwallace-microsoft]

Description

Restructure the .NET samples into a clear progressive learning path with 5 sections:

• 01-get-started/ — 6 numbered steps (hello agent → hosting), each adding one concept
• 02-agents/ — Deep-dive concept samples: tools/, middleware/, conversations/, providers/
• 03-workflows/ — One folder per workflow pattern (sequential, concurrent, branching, HITL, etc.)
• 04-hosting/ — Multi-project solutions: a2a, ag-ui, azure-functions, openai-endpoints
• 05-end-to-end/ — Complete apps: agent-web-chat, agui-web-chat, m365-agent, purview
• Add integration tests so we can automating testing of as many samples as possible

Key design decisions

• Must align with the Python samples and documentation changes
• All documentation samples must reference samples in the main repo (already done for Python), there is a skill in the docs PR to set this up
• One concept per file — each .cs demonstrates a single topic
• Snippet tags — all samples include // / // tags for docs :::code references
• Explicit env vars — Environment.GetEnvironmentVariable() with consistent variable names between all samples, integrations tests
• Default provider — Azure AI Foundry via AIProjectClient
• Single-file for 01-03 — top-level statements; 04/05 are multi-project solutions
• AGENTS.md — documents structure, conventions, and file→docs mapping for future maintainers

Reference PRs

Python Samples:
#3765
#3862

Python Docs:
https://github.com/MicrosoftDocs/semantic-kernel-pr/pull/858

.NET Sample Updates:
#3978

[crickman]

Feature Branch - #4092
Configuration Updates - #4149