[Agents Extension] Unable to generate bicep into existing project

[trangevi]

Starting from an existing code project which already has bicep, we error out when a user attempts to eject the bicep which we would need/use for our resources if it's in the same location

ERROR: `./infra/` already exists
 
Suggestion: to regenerate from azure.yaml, delete the infra directory and run the command again

We should likely attempt to merge the directories, prompting if we run into conflicts

[hund030]

I think file-level merging may not be sufficient because infra/main.bicep will usually conflict and is the deployment entry point. Resource-level Bicep merging would also be complex and risky.

We may need to use infra.layers instead: preserve the existing Bicep as one layer and generate Foundry infrastructure into a separate layer such as infra/foundry. This avoids modifying or semantically merging the user’s Bicep.

This would require the extension to honor layer-specific path and module settings instead of hardcoding infra/main.bicep. We should also treat conversion of an existing single-layer configuration as an explicit migration because it may affect preview, hooks, dependencies, and older azd versions.