.NET: Foundry V2 - Update Sample and Docs for clear distinction between Classic/New Foundry Agents.

[rogerbarreto]

This pull request updates the Azure Foundry Agents samples and documentation to clarify the differences between the classic and new Foundry Agents experiences, and to improve code clarity and consistency in the agent creation samples. The changes also update variable names for better readability and remove unnecessary constants.

Documentation improvements:

• Added sections in the README.md files for both classic and new Foundry Agents samples, including a comparison and a link to the official migration guide. [1] [2] [3]
• Updated the main Foundry Agents README.md to highlight the new experience and direct users to the appropriate samples and migration documentation.

Sample code consistency and clarity:

• In both Agent_With_AzureAIProject/Program.cs and FoundryAgents_Step01.1_Basics/Program.cs, removed unnecessary instruction string constants, updated variable names (e.g., jokerAgentV1 → existingJokerAgent, jokerAgentV2 → newJokerAgent, latestVersion → latestAgentVersion), and inlined instruction strings for improved clarity. [1] [2]
• Updated agent deletion code to use the new variable names for consistency. [1] [2]

[Copilot]

Pull request overview

This PR clarifies the distinction between classic and new Azure Foundry Agents experiences by updating documentation and improving code clarity in sample files. The changes help developers understand which agent type they're working with and provide guidance for migration.

Key changes:

• Added documentation sections explaining classic vs new Foundry Agents with migration guide links
• Improved variable naming (e.g., jokerAgentV1 → existingJokerAgent, latestVersion → latestAgentVersion)
• Removed unnecessary instruction constants and inlined literal strings for improved clarity

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
dotnet/samples/GettingStarted/FoundryAgents/README.md	Added section explaining classic vs new Foundry Agents with migration documentation link
dotnet/samples/GettingStarted/AgentProviders/Agent_With_AzureAIProject/README.md	Added header and comparison section identifying this as a new Foundry Agents sample
dotnet/samples/GettingStarted/AgentProviders/Agent_With_AzureAIProject/Program.cs	Updated variable names, removed constant, and inlined instruction strings for clarity
dotnet/samples/GettingStarted/AgentProviders/Agent_With_AzureAIAgentsPersistent/README.md	Added header and comparison section identifying this as a classic Foundry Agents sample
dotnet/samples/GettingStarted/FoundryAgents/FoundryAgents_Step01.1_Basics/Program.cs	Updated variable names, removed constants, and inlined instruction strings for clarity