Address CI errors

[bpark1327]

Closes #137

[bpark1327]

Root Cause:
eeweather's cache.py calls os.makedirs(directory) with the default exist_ok=False. When tests run in parallel via pytest-xdist (as indicated by the [gw1] marker in the logs), multiple workers race to create /home/runner/.eeweather at the same time. The second worker to arrive raises a FileExistsError.

Proposed Fix:
Isolate the eeweather cache directory per test worker by overriding the cache path in conftest.py

[bpark1327]

Root Cause:
matplotlib automatically selects an interactive backend at import time based on the runtime environment. In headless Linux CI environments, this selection fails because there is no active display session.

Proposed Fix:
Added a session-scoped fixture in conftest.py that forces matplotlib to use the Agg backend (a non-interactive, file-based renderer) before any tests run

[shorowit]

Seems to be working nicely. I triggered a second CI run and that was also successful. Thanks @bpark1327! I still think there could be an issue that our results are not always reproducible/deterministic, but if we see that again from the CI, we can open a separate issue.