text: Title: DataArray.get_axis_num is incorrectly typed
  Body: ### What happened?
  
  When using mypy to check my code, I'm getting a spurious error. When passing a single string to `get_axis_num`, the return type is determined to be `tuple[int, ...]` instead of `int`.
  
  ### What did you expect to happen?
  
  I expected the return type to be `int` when passing a single single.
  
  ### Minimal Complete Verifiable Example
  
  ```Python
  # Note: to be run through mypy
  import xarray as xr
  
  def foo(x: xr.DataArray) -> None:
      reveal_type(x.get_axis_num("time"))
  ```
  
  
  ### MVCE confirmation
  
  - [X] Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
  - [X] Complete example — the example is self-contained, including all data and the text of any traceback.
  - [ ] Verifiable example — the example copy & pastes into an IPython prompt or [Binder notebook](https://mybinder.org/v2/gh/pydata/xarray/main?urlpath=lab/tree/doc/examples/blank_template.ipynb), returning the result.
  - [X] New issue — a search of GitHub Issues suggests this is not a duplicate.
  - [X] Recent environment — the issue occurs with the latest version of xarray and its dependencies.
  
  ### Relevant log output
  
  ```Python
  # Output from mypy
  test.py:4: note: Revealed type is "builtins.tuple[builtins.int, ...]"
  Success: no issues found in 1 source file
  ```
  
  
  ### Anything else we need to know?
  
  From a quick look at the code, I'm guessing this is because `str` is iterable and so is matching the first `@overload`.
  
  ### Environment
  
  <details>
  
  
  INSTALLED VERSIONS
  ------------------
  commit: None
  python: 3.12.3 (main, Nov  6 2024, 18:32:19) [GCC 13.2.0]
  python-bits: 64
  OS: Linux
  OS-release: 6.8.0-49-generic
  machine: x86_64
  processor: x86_64
  byteorder: little
  LC_ALL: None
  LANG: C.UTF-8
  LOCALE: ('C', 'UTF-8')
  libhdf5: 1.14.2
  libnetcdf: None
  
  xarray: 2024.11.0
  pandas: 2.1.4
  numpy: 1.26.2
  scipy: 1.11.4
  netCDF4: None
  pydap: None
  h5netcdf: None
  h5py: 3.11.0
  zarr: None
  cftime: None
  nc_time_axis: None
  iris: None
  bottleneck: None
  dask: 2023.12.0
  distributed: None
  matplotlib: 3.9.1
  cartopy: None
  seaborn: None
  numbagg: None
  fsspec: 2023.12.1
  cupy: 13.2.0
  pint: None
  sparse: None
  flox: None
  numpy_groupies: None
  setuptools: 75.1.0
  pip: 24.3.1
  conda: None
  pytest: 8.2.2
  mypy: 1.11.1
  IPython: 8.18.1
  sphinx: 7.2.6
  
  </details>
  