Add --region option and tests

[Ayush-Devs]

Hi maintainers,

This PR is a work-in-progress draft to improve the CLI experience for running GRASS tools. It includes the following changes:

Added --region support to the run subcommand:
Allows users to temporarily set a computational region (e.g., raster map) before running a tool.

Initial tests for subcommand_run_tool using mocks:
test_run_tool_with_region checks that g.region is called when --region is provided.
test_run_tool_without_region ensures g.region is not called when the option is omitted.
All tests mock GRASS binaries to avoid requiring a full GRASS session.

Notes / Guidance Requested.

The mocks allow the tests to run without setting up a full GRASS environment, but I’m not sure if this aligns with project standards.
Should I replace these with integration-style tests once a GRASS session is available?
Any recommendations on how to structure or extend these tests for coverage of other CLI features would be greatly appreciated.

This PR is explicitly draft/WIP; the goal is to get feedback on the approach before finalizing tests or merging.

Verified both test_run_tool_with_region and test_run_tool_without_region run successfully with mocks.
Existing CLI tests (--help, man, regular tool runs) continue to pass.

Next steps (after feedback)

Potentially add full integration tests using a live GRASS session.
Refactor mocks if needed.
Expand tests for other flags or tools in the CLI.

[wenzeslaus]

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