[sumitchintanwar]

This PR adds comprehensive tests for the map reuse functionality. As asked in PR #6953.

The reuse functionality reuses the calculated maps instead of recalculating them again, which saves time and a lot of computational power.

The PR adds three classes of tests:

1. Basic reuse workflow
2. Input validation
3. Performance testing (Skipped in CI)

Tests use @unittest.expectedFailure decorator since accumulation_input and drainage_input parameters are not yet implemented. This allows tests to run in CI without failing the build. Once that feature is merged, The @unittest.expectedFailure will be removed in a follow up commit.

Verification: All tests run successfully locally with the implemented changes, passing with 0 errors

Also, Tests for TCI/SPI calculation and basin delineation with reused maps are not included in this PR, as these features require significant algorithm changes and will be addressed in separate follow-up work once the core map reuse functionality is stable.

[echoix]

Now needs SPDX header

[Valyrian-Code]

#6953 is approved but unmerged since April, so accumulation_input and drainage_input are not in main. I ran the file: exit 0, 4 tests, 2 skipped and 2 expected failures, so nothing here checks anything until that lands. And once those two do start passing, expectedFailure turns them into unexpected successes, which exits 1.

New tests go in pytest now too, though that landed in August, after you opened this.

Smaller things

where	what
TestWatershedReuseLarge	Skipped on the class and again on its only test.
rand(0, 100)	No seed. Ran it twice, got different maps.
length_slope	Only assertRasterExists, never compared.
docstring	Says r_watershed_reuse_test, file is test_r_watershed_reuse.py.