[BUG] pandas mismatch for `groupby(...).transform('size')`

**Describe the bug**

There's a few issues here:
- the return type of `transform('size')` is `DataFrame`, whereas in pandas it would be `Series`
- `transform('size')` raises if there are string columns in the dataframe (even if they're not being grouped by)

**Steps/Code to reproduce bug**
Follow this guide http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports to craft a minimal bug report. This helps us reproduce the issue you're having and resolve the issue more quickly.

```python
import cudf

df = cudf.DataFrame({"a": [1, 2, 2], "b": [4,5,6]})
print(df.groupby('a').transform('size'))
```
outputs
```
b
0 1
1 2
2 2
```

**Expected behavior**
pandas does
```
0 1
1 2
2 2
dtype: int64
```

**Environment overview (please complete the following information)**
- Environment location: [Bare-metal, Docker, Cloud(specify cloud provider)]
- Method of cuDF install: [conda, Docker, or from source]
- If method of install is [Docker], provide `docker pull` & `docker run` commands used

cudf version: '25.04.00'
pandas version: 2.2.3

**Environment details**
Please run and paste the output of the `cudf/print_env.sh` script here, to gather any other relevant environment details

**Additional context**

spotted in [narwhals](https://github.com/narwhals-dev/narwhals)