text: Title: BUG: Single Index of Tuples as Output on Tuple Groupings
  Body: ### Pandas version checks
  
  - [X] I have checked that this issue has not already been reported.
  
  - [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.
  
  - [X] I have confirmed this bug exists on the [main branch](https://pandas.pydata.org/docs/dev/getting_started/install.html#installing-the-development-version-of-pandas) of pandas.
  
  
  ### Reproducible Example
  
  ```python
  df = pandas.DataFrame({"A" : [1,2,3,1,2,3], "B": [4,5,6,4,5,6], "C" : [7,8,9,7,8,9]})
  df = df.set_index(['A', 'B'])
  df.groupby(lambda x : (x[0], x[1])).aggregate('sum')
  ```
  
  
  ### Issue Description
  
  This issue appears to be similar to #24786 except that here a lambda is used to create the tuples. The [User Guide for Pandas](https://pandas.pydata.org/docs/user_guide/groupby.html) states that 
  > The result of the aggregation will have the group names as the new index. In the case of multiple keys, the result is a MultiIndex by default.
  
  Thus I was expecting the tuples to be automatically combined to form a ```MultiIndex``` as opposed to an ```Index``` with tuples as indices. Internally, the ```Index.map()``` [call](https://github.com/pandas-dev/pandas/blob/8a5344742c5165b2595f7ccca9e17d5eff7f7886/pandas/core/groupby/grouper.py#L511) converts the produced list of tuples to a ```MultiIndex```, but when it [produces the aggregation index](https://github.com/pandas-dev/pandas/blob/8a5344742c5165b2595f7ccca9e17d5eff7f7886/pandas/core/groupby/ops.py#L753), it appears to produce a single ``Index``` instead.
  
  I wasn't sure if this was intended behaviour, or a bug; I apologize if it is the former!
  
  ### Expected Behavior
  
  ```python
  df = pandas.DataFrame({"A" : [1,2,3,1,2,3], "B": [4,5,6,4,5,6], "C" : [7,8,9,7,8,9]})
  df = df.set_index(['A', 'B'])
  df.groupby(['A', 'B']).aggregate('sum')
  ```
  
  ### Installed Versions
  
  <details>
  
  INSTALLED VERSIONS
  ------------------
  commit                : 0691c5cf90477d3503834d983f69350f250a6ff7
  python                : 3.13.1
  python-bits           : 64
  OS                    : Windows
  OS-release            : 10
  Version               : 10.0.18363
  machine               : AMD64
  processor             : AMD64 Family 21 Model 96 Stepping 1, AuthenticAMD
  byteorder             : little
  LC_ALL                : None
  LANG                  : None
  LOCALE                : English_Canada.1252
  
  pandas                : 2.2.3
  numpy                 : 2.2.1
  pytz                  : 2024.2
  dateutil              : 2.9.0.post0
  pip                   : 24.3.1
  Cython                : None
  sphinx                : None
  IPython               : None
  adbc-driver-postgresql: None
  adbc-driver-sqlite    : None
  bs4                   : None
  blosc                 : None
  bottleneck            : None
  dataframe-api-compat  : None
  fastparquet           : None
  fsspec                : None
  html5lib              : None
  hypothesis            : None
  gcsfs                 : None
  jinja2                : None
  lxml.etree            : None
  matplotlib            : None
  numba                 : None
  numexpr               : None
  odfpy                 : None
  openpyxl              : None
  pandas_gbq            : None
  psycopg2              : None
  pymysql               : None
  pyarrow               : None
  pyreadstat            : None
  pytest                : None
  python-calamine       : None
  pyxlsb                : None
  s3fs                  : None
  scipy                 : None
  sqlalchemy            : None
  tables                : None
  tabulate              : None
  xarray                : None
  xlrd                  : None
  xlsxwriter            : None
  zstandard             : None
  tzdata                : 2024.2
  qtpy                  : None
  pyqt5                 : None</details>
  