Rename Agent to AIAgent and add DisplayName prop

[westey-m]

Motivation and Context

To avoid naming clashes we are renaming Agent to AIAgent.
This matches naming in MEAI.

Description

• Rename Agent to AIAgent
• Add new DisplayName property on AIAgent that returns Name and falls back to ID.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[Copilot]

Pull Request Overview

This PR renames the Agent class to AIAgent throughout the codebase to avoid naming clashes and adds a new DisplayName property. The renaming aligns with MEAI naming conventions while the new property provides a fallback from Name to Id for display purposes.

Key changes:

• Rename Agent class to AIAgent across all files
• Add DisplayName property that returns Name ?? Id
• Update all references, inheritance, parameters, and documentation

Reviewed Changes

Copilot reviewed 37 out of 37 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Extensions.AI.Agents.Abstractions/AIAgent.cs	Core class rename and DisplayName property addition
dotnet/src/Microsoft.Extensions.AI.Agents/OpenTelemetryAgent.cs	Updated to inherit from AIAgent and accept AIAgent parameters
dotnet/src/Microsoft.Extensions.AI.Agents/ChatCompletion/ChatClientAgent.cs	Updated inheritance to extend AIAgent
dotnet/src/Microsoft.Extensions.AI.Agents/AgentExtensions.cs	Updated extension methods to work with AIAgent
dotnet/src/Microsoft.Extensions.AI.Agents.CopilotStudio/CopilotStudioAgent.cs	Updated inheritance to extend AIAgent
Multiple orchestration files	Updated to use AIAgent in constructors, parameters, and field types
Multiple test files	Updated mock objects, type references, and fixture implementations
Sample files	Updated agent instantiation and usage examples