.Net: Add an agent RAG sample, add text only upload, bug fixes and more tests

[westey-m]

Motivation and Context

#10100
#10712

Description

• Adds a sample that shows how to use the rag components with an agent
• Adds support for text only uploads
• Adds some bug fixes
• Adds additional tests

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄