External modules cannot wrap itkPyBuffer.h: Python include dir is not exported

An external module that wraps `itkPyBuffer.h` cannot find `Python.h`: the Python include directory is never exported, so castxml fails outside ITK's own build tree.

## Reproduce

Build an external module that wraps `itkPyBuffer.h` (RTK does, via
`wrapping/itkPyBufferRTK.wrap`) against an ITK built with `ITK_WRAP_PYTHON=ON`:

```
FAILED: .../Wrapping/castxml_inputs/itkPyBufferRTK.xml
.../ITK/Modules/Bridge/NumPy/include/itkPyBuffer.h:30:10: fatal error: 'Python.h' file not found
30 | #include
```

The same target succeeds with the include supplied by hand:

```bash
CPATH="$CONDA_PREFIX/include/python3.14" ninja .../itkPyBufferRTK.xml # OK
```

## Cause

`Wrapping/Generators/Python/CMakeLists.txt:20` adds the Python include as a
**directory property**:

```cmake
include_directories("${Python3_INCLUDE_DIRS}")
```

and `Wrapping/Generators/CMakeLists.txt:99-104` re-reads that property and
re-applies it. Both act only inside ITK's own build tree.

An external module generates its `*.castxml.inc` from
`WRAPPER_LIBRARY_INCLUDE_DIRECTORIES` plus each dependency's
`INTERFACE_INCLUDE_DIRECTORIES`
(`Wrapping/macro_files/itk_auto_load_submodules.cmake:107,127,131`). The
exported `ITKBridgeNumPy.cmake` carries no Python include — its only Python
entry is `ITKBridgeNumPy_RUNTIME_LIBRARY_DIRS` — so the directory is absent
from the generated include list.

Measured on one such build: ITK's own `itkPyBuffer.castxml.inc` has 25 include
directories including `.../include/python3.14`; the external module's
`itkPyBufferRTK.castxml.inc` has 109 directories and none of them is Python's.

## Effect

The castxml step fails, wrapping never completes, and the module leaves a
`Config.py` in ITK's Python package with no `Python.py` and no
`_Python*.so`. Every later `import itk` in that build tree then raises

```
FileNotFoundError: .../itk/support/../RTKPython.py
```

while the lazy loader walks the factories, so unrelated consumers' Python tests
fail too.

## Possible directions

- Export the Python include with `ITKBridgeNumPy`'s interface, so any module
that consumes it inherits the directory.
- Or add `Python3_INCLUDE_DIRS` to the include list used to generate
`*.castxml.inc` for external modules.

Related in shape to #6860, where a wrapping/factory input also resolves only
inside ITK's own build tree.

Start with Wrapping/Generators/Python/CMakeLists.txt, Wrapping/Generators/CMakeLists.txt, and Wrapping/macro_files/itk_auto_load_submodules.cmake to trace how Python3_INCLUDE_DIRS reaches generated castxml include lists and exported interfaces. Reproduce the RTK external wrapping case with ITK_WRAP_PYTHON=ON and inspect the generated *.castxml.inc files. Done means an external wrapper can find Python.h, complete castxml generation, and produce its Python module artifacts without CPATH.

Written by the indexing model from the issue text.