Fix: Store price calculations on small file

[1yam]

For store message we used a SizedVolume who have size_mib: int.
for file of example 14 bytes when it will be converted to mib it's will be really small number and it's will be be round at 0

Self proofreading checklist

• Is my code clear enough and well documented
• Are my files well typed
• New translations have been added or updated if new strings have been introduced in the frontend
• Database migrations file are included
• Are there enough tests
• Documentation has been included (for new feature)

Changes

This pull request focuses on improving the precision of volume size calculations by changing the type of size_mib from int to Decimal across various functions and classes in the aleph codebase. Additionally, it includes related updates to tests to ensure the new type is handled correctly.

Changes to improve precision:

• src/aleph/services/cost.py: Updated multiple functions (_get_volumes_costs, _get_execution_volumes_costs, _calculate_storage_costs) to convert volume sizes to Decimal for more accurate calculations. [1] [2] [3] [4] [5]

Type change in class definition:

• src/aleph/types/cost.py: Modified the SizedVolume class to use Decimal for size_mib instead of int.

Updates to tests:

• tests/message_processing/test_process_stores.py: Added Decimal import and updated tests to check the new Decimal type for volume sizes. Included a new test to assert the cost calculation using the Decimal type. [1] [2] [3]

[Psycojoker]

LGTM