Fix FileFeatureStorage path resolution on case-sensitive filesystems

[pdwi2020]

Summary

• keep the existing lowercase path as default for feature storage
• add case-aware fallback that resolves existing instrument directories/files by case-insensitive match
• add a regression test for uppercase instrument directory (ABB-U) with close.day.bin

Why

This fixes loading failures on case-sensitive filesystems when data is stored under uppercase instrument folders, as reported in #2053.

Fixes #2053

Testing

• python3 -m py_compile qlib/data/storage/file_storage.py tests/storage_tests/test_storage.py
• Full pytest target requires compiled qlib C extensions in this environment.

[pdwi2020]

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