test: add bats tests for repo_root

[RodrigoEspinosa]

🎯 What: Added tests for the repo_root bash function located in bin/wt to fill the testing gap.
📊 Coverage: Covered the happy path (when git rev-parse --show-toplevel succeeds) and fallback paths (when it fails or returns empty, falling back to git rev-parse --git-dir). Tests use BATS and mock git output correctly.
✨ Result: Improved test coverage and reliability for path resolution using git commands, validating all possible outputs.

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PR created automatically by Jules for task 13735739386434642129 started by @RodrigoEspinosa

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