FD limit is hardcoded to 1024

https://github.com/boxlite-ai/boxlite/pull/406

```
/home/xhe/project/boxlite/src/boxlite/src/runtime/advanced_options.rs
env_allowlist: vec!["RUST_LOG".to_string()],
resource_limits: ResourceLimits {
max_open_files: Some(1024),
```

Mounting a large shared directory will soon reach the limit by `virtio-fs`. Guest may have problems, too.

This should be configurable.

Start in src/boxlite/src/runtime/advanced_options.rs, where ResourceLimits sets max_open_files to Some(1024). Trace how runtime options are supplied and identify the existing configuration path for exposing this limit. Done means callers can configure the file-descriptor limit instead of using the hardcoded value, including a suitable validation or regression check if the surrounding code provides one.

Written by the indexing model from the issue text.