Add pytest markers to separate fast tests from network/credential test

[toakinrele]

Closes #115

Adds network and aws_creds pytest markers so the fast unit tests can be run separately from tests that need network access or AWS/GCS credentials.

• Registered markers in pyproject.toml
• conftest.py auto-tags tests based on which fixtures they use (download_gpkg, clean_s3_test, etc.), and auto-skips aws_creds
  tests when no AWS credentials are detected
• Explicitly marked the handful of tests that hit the network directly without a fixture (test_hf2ds.py, some of test_trouterestarts.py)
• Added a Makefile with test-fast / test-network / test-all targets

Local fast loop: make test-fast - 7 tests in ~3s instead of the full ~30 min suite, no credentials required.

[quinnylee]

This is cool. I wonder if it would be possible to tease out some unit tests from all the integration tests that we have skipped? The fast test suite doesn't run anything from test_channelrouting.py, test_forcingprocessor.py, test_hf2ds.py, test_nrds_fp.py, or test_plotter.py

[quinnylee]

This is neat!

[toakinrele]

For the five files you named, the tests are mostly thin wrappers (call hf2ds()/prep_ngen_data() with a real source, assert a file landed). The network call and the computation are fused inside those functions, so there's no clean pure logic to pull out without refactoring the source itself.

I did find three pure functions already sitting in processor.py (distribute_work, load_balance, calculate_vpu_precip_stats) that were only ever tested indirectly. Added direct unit tests for them, fast suite is now 18 tests, still ~3s. I also found a real bug in load_balance (it mishandles the all-zero-work case), so I marked it xfail with an explanation rather than fixing it here. (c6305c3)

[quinnylee]

Maybe this is something we can delay until we merge the refactor branch?

[quinnylee]

Thanks for the work! This topic (unit tests) will need to get revisited once we figure out what to do with the refactoring branch.