Feature Request: Enable XDebug on multiple sites simultaneously

Based on [this user's feedback](https://x.com/davidperezmk/status/2075101411671498867)

### What

Allow XDebug to be activated on more than one site at the same time.

### Why

Currently XDebug can only be active on one site at a time. Developers working across multiple interconnected sites (e.g., multisite, API integrations) need to constantly toggle XDebug between sites, which disrupts the debugging workflow.

### How

- Allow concurrent XDebug sessions across multiple Studio sites.
- Each site's PHP process could bind to a different debug port.

Start by tracing how Studio currently activates XDebug for one site and how each site's PHP process receives its debug configuration. Verify the behavior with multiple interconnected sites, then confirm that XDebug can remain active concurrently and that each PHP process uses a distinct debug port.

Written by the indexing model from the issue text.