.NET: Add hosted session and delegated user identity samples #8226

Hosted agent client samples do not currently demonstrate two important Foundry runtime patterns:

• Creating a hosted session explicitly, attaching it to an Agent Framework session, and managing its lifecycle.
• Sharing a bounded hosted session pool across application users while delegating a distinct user identity on each call.

Add runnable .NET client samples under FoundryHostedAgents/responses/Using-Samples for both scenarios. Document the distinction between hosted sessions and conversations, session pooling, delegated identity permissions, and application owned data partitioning.

Also remove obsolete manual Foundry-Features headers from samples now covered by the current Foundry API and SDK behavior.