.NET: Add Foundry SharePoint Tool Sample

[rogerbarreto]

Motivation and Context

Closes #3674

Adds a new FoundryAgents sample demonstrating how to use the SharePoint grounding tool with AI Agents. This enables agents to search and retrieve information from SharePoint sites via Azure Foundry.

Description

Adds FoundryAgents_Step22_SharePoint sample with:

• Two agent creation approaches (consistent with other tool samples):
  • Option 1 (MEAI): Using AgentTool.CreateSharepointTool() + AsAITool() wrapper
  • Option 2 (Native SDK): Using PromptAgentDefinition with native AgentTool.CreateSharepointTool()
• SharePoint grounding tool configuration via SharePointGroundingToolOptions and project connections
• Display of grounding annotations from SharePoint search results
• Agent lifecycle management (creation and cleanup)

Files added/modified:

• Program.cs — Sample implementation with both MEAI and Native SDK approaches
• FoundryAgents_Step22_SharePoint.csproj — Project file targeting net10.0
• README.md — Sample documentation with prerequisites and usage instructions
• FoundryAgents/README.md — Updated samples table with Step 22 entry

Validated end-to-end against agents-e2e-tests-westus2 project with gpt-4o deployment.

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.

[Copilot]

Pull request overview

This PR adds a new SharePoint grounding sample (Step 22) to the FoundryAgents getting started samples. The sample demonstrates how to use the SharePoint grounding tool to search and retrieve information from SharePoint sites within AI agents.

Changes:

• Added a new sample directory FoundryAgents_Step22_SharePoint with Program.cs, README.md, and project file
• Updated the main FoundryAgents README.md to include the new SharePoint sample in the table of contents
• Demonstrates two agent creation approaches: using MEAI abstraction and native SDK

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
dotnet/samples/GettingStarted/FoundryAgents/README.md	Added reference to the new SharePoint grounding sample in the samples index
dotnet/samples/GettingStarted/FoundryAgents/FoundryAgents_Step22_SharePoint/README.md	Documentation for the SharePoint grounding sample including prerequisites, setup, and expected behavior
dotnet/samples/GettingStarted/FoundryAgents/FoundryAgents_Step22_SharePoint/Program.cs	Implementation showing SharePoint grounding tool usage with two agent creation patterns
dotnet/samples/GettingStarted/FoundryAgents/FoundryAgents_Step22_SharePoint/FoundryAgents_Step22_SharePoint.csproj	Project configuration with required dependencies and compiler settings