Dropping a folder on the file uploader reports a file failure that did not happen

### What happened?

Dropping a folder onto the dataset file uploader reports a file failure that did not happen.

In `files-uploader.component.ts`, a dropped entry that is not a file resolves to `null`:

```ts
} else {
resolve(null); // directory
}
```

Those nulls are then filtered out of `successfulUploads`, but the failure count is computed against the *unfiltered* results:

```ts
const failedCount = results.length - successfulUploads.length;
if (failedCount > 0) {
this.showFileUploadBanner("error", `${failedCount} file${failedCount > 1 ? "s" : ""} failed to be selected.`);
}
```

A directory therefore counts as a failed file, even though nothing failed and nothing was meant to be uploaded.

### How to reproduce?

1. Open a dataset and drag a **folder** onto the file drop area (alone, or alongside a valid file).
2. A red banner appears: *"1 file failed to be selected."*
3. Any valid files dropped alongside it are still selected correctly — only the banner is wrong.

### Version/Branch

1.3.0-incubating-SNAPSHOT (main)

### Expected behavior

A dropped directory is not a failure. `failedCount` should count only entries that actually rejected, i.e. exclude the deliberate `resolve(null)` cases — for example by tracking rejections separately, or by counting `results.filter(r => r.status === "rejected").length` rather than deriving from the length difference.

### Additional context

Found while adding coverage for the drop path. The accompanying test asserts only that the directory is excluded from the selection and deliberately does **not** assert the banner, so the miscount is not cemented while it stands.