feat: Add GlobalDummyDatastore for global domain testing

[sohampatil01-svg]

Describe your changes

Introduces a GlobalDummyDatastore test variant that simulates a global domain by returning an all-zero boundary mask. This enables testing models without lateral boundary forcing. Includes a new end-to-end training test case.

Issue Link

Closes #444

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 Documentation (Addition or improvements to documentation)

Checklist before requesting a review

• My branch is up-to-date with the target branch
• I have performed a self-review of my code
• For any new/modified functions/classes I have added docstrings that clearly describe its purpose, expected inputs and returned values
• I have placed in-line comments to clarify the intent of any hard-to-understand passages of my code
• I have updated the README to cover introduced code changes
• I have added tests that prove my fix is effective or that my feature works
• I have given the PR a name that clearly describes the change, written in imperative form.

[joeloskarsson]

Duplicate of #444, which you yourself link to 😓