Inability to Leverage Existing Azure OpenAI (AOAI) Resources in the new Azure AI Foundry setup for Agents

**Is your feature request related to a problem? Please describe.**
The Azure AI Foundry platform provides a method to connect to existing Azure OpenAI (AOAI) resources, which is great for resource governance. However, the documentation and user experience for the Agents service do not clearly explain how to utilize these connected resources. When building an agent, the default or seemingly required behavior is to use the AOAI resource created as part of the AI Foundry project. This ambiguity forces developers to either duplicate resources or spend significant time searching for an undocumented method to leverage their existing, linked AOAI deployments for agent creation, hindering productivity

**Describe the solution you'd like**
Would like to see support for using existing AOAI resource for agent service. Specifically, the agent creation workflow should have a clear option to select from existing, connected AOAI resources both via portal and SDK. The documentation should be updated with a step-by-step guide on how to configure an agent to use a pre-existing AOAI resource.

**Describe alternatives you've considered**
I have explored using the default, project-specific AOAI resource created by AI Foundry, but this is not a viable option for our customer due to the need for centralized resource management, cost allocation, and quota sharing across multiple projects. I have also attempted to find a way to manually configure the agent's connection to an existing AOAI resource through various settings on the portal and SDK, but the process is not clearly documented, leading to trial and error.

**Additional context**
Add any other context or screenshots about the feature request here.

Inizia esaminando il workflow di creazione degli agenti di Azure AI Foundry nel portale e nell'SDK, concentrandoti sul modo in cui vengono rappresentate le risorse Azure OpenAI esistenti e connesse. Individua quindi i punti di ingresso rilevanti per la configurazione dell'agente e le posizioni della documentazione. Il lavoro sarà completato quando gli utenti potranno selezionare una risorsa connessa esistente sia tramite il portale sia tramite l'SDK, con una guida alla configurazione passo passo.

Scritto dal modello di indicizzazione a partire dal testo della issue.