fix(mine): fallback to test pool when assign_splits hashes all tasks to test

[RohithPariki]

Fixes #271.

When all real tasks hash to the \ est\ split, the \�ssign_splits\ function previously failed to satisfy its guarantees for non-empty \�al\ and \ rain\ splits, as the promotion logic strictly refused to pull from \ est. This silently resulted in \�al\ and \ rain\ both being empty.

This pull request implements the requested fallback:

• Reverts to using \ est\ tasks when both \ rain\ and \�al\ are entirely empty.
• Avoids demoting hash-assigned \ est\ tasks whenever possible by keeping existing source preferences.
• Emits a logged warning through \logging.getLogger('skillopt_sleep').warning\ detailing the fallback and parameters.
• Adds a robust unit test to \ ests/test_split_hardening_2x3.py\ ensuring the fallback mechanism appropriately staffs the \�al\ and \ rain\ sets in this edge case without altering behavior in normal cases.