Tests: add pytest-based test for rasterExists helper function

[ayushjariyal]

This PR introduces a pytest-based test for rasterExists and a fixture for automatic GRASS session setup.

Changes:

Added assertions.py implementing rasterExists
Added conftest.py with a pytest fixture for GRASS session setup
Added test_assertions.py containing pytest-based tests

Testing:

GSoC 2026:

This contribution is a part of GSoC 2026 Support writing tests with pytest project.

[ayushjariyal]

@petrasovaa @wenzeslaus

While working on the pytest conversion, I organized the related utilities into a new directory python/grass/pytest to keep the code cleaner and allow easier expansion of pytest-related functionality in the future.

However, the CI checks are currently failing with the following error:

ModuleNotFoundError: No module named 'grass.pytest'

It seems that the new subpackage may not be included in the installed GRASS Python package used in CI.

Before proceeding further, I wanted to confirm whether introducing a separate submodule like grass.pytest is acceptable for the project structure, or if it would be preferable to keep these utilities within the existing grass module.

If the separate submodule approach is acceptable, could you please suggest what changes might be needed so that grass.pytest becomes available during CI builds?

I would like to follow the structure that fits best with the project.

[echoix]

Most probably the makefiles and CMakeLists need to be adjusted in consequence. Look for the closest ones up in the folder hierarchy

[ayushjariyal]

@echoix, @petrasovaa , Now all the checks are passed. Could you please review it now?

[wenzeslaus]

This is a lot of code for what are basically two lines of new code. Yes, something like special set of functions to aid pytest might be needed, but let's think about this first: Raster existence check goes beyond pytest-based testing. Where it can live instead? Can the grass.gunittest code use such a function?

This is also a lot of noise compared to the resulting code. While the current code has some merit, it is nowhere close to being merged.

[ayushjariyal]

@wenzeslaus , Thanks for the feedback!

I’ve updated the helper to behave as a proper pytest-style assertion by raising AssertionError instead of returning a boolean. I also renamed it to raster_exists to follow snake_case conventions.
Test have been updated to use pytest.raises for failure cases.
Please review and let me know if any changes are needed.