fix(storage): Fix FileFeatureStorage path resolution on case-sensitive file systems

[anish-devgit]

Summary

Fixes #2053: FileFeatureStorage incorrectly lowercases instrument directory names, causing failures on case-sensitive file systems.

What I changed

• Updated qlib/data/storage/file_storage.py to resolve instrument directories using the correct filesystem case when available.
• Added a fallback to lowercase paths for backward compatibility.
• Ensured new file creation preserves the original instrument case.
• Added tests/storage_tests/test_issue_2053.py to validate behavior on case-sensitive systems.

How I tested

• Added an isolated test (test_issue_2053.py) that reproduces the issue by creating an uppercase instrument directory.
• Verified that FileFeatureStorage resolves paths correctly after the fix.
• Note: Full test suite was not executed locally due to heavy optional dependencies; CI should validate integration.

Before / After

Before
FileFeatureStorage(instrument="AAPL").uri → .../aapl/close.day.bin (fails if directory is AAPL)

After
FileFeatureStorage(instrument="AAPL").uri → .../AAPL/close.day.bin

Fixes #2053

[anish-devgit]

@microsoft-github-policy-service agree

[SunsetWolf]

Hi @anish-devgit , Thank you so much for taking the time to contribute and for providing a thoughtful solution! We really appreciate your effort.

In this case, Qlib intentionally normalizes instrument names to lowercase to ensure consistent behavior across different platforms, especially Windows. Allowing multiple case variants in FileFeatureStorage would break that consistency and could introduce subtle cross-platform issues.

Thanks again for the contribution and for offering to submit a PR — we really value the discussion and grateful for your understanding.

[anish-devgit]

Thanks a lot for the detailed explanation and for reviewing my PR 🙏
That makes sense — I understand the design decision around normalizing to lowercase for cross-platform consistency.

Appreciate the guidance and the kind feedback. Happy to contribute again in the future!