fix: replace Windows backslashes with forward slashes in paths

[yunchipang]

Description

Write util functions to replace backslashes with forward slashes when constructing file paths.

fixes: #445

How are the changes test-covered

• [ ] N/A
• [x] Automated tests (unit and/or integration tests)
• [x] Manual tests (OS: Windows 11, Python: 3.13.7, in real AWS S3 env)
  • [ ] Details are described below

[codecov[bot]]

Codecov Report

:x: Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review. :white_check_mark: Project coverage is 86.85%. Comparing base (038eca8) to head (f8f7307). :warning: Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/core/src/schema/resolver.rs	50.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #457      +/-   ##
==========================================
+ Coverage   86.72%   86.85%   +0.12%     
==========================================
  Files          49       49              
  Lines        2637     2625      -12     
==========================================
- Hits         2287     2280       -7     
+ Misses        350      345       -5     

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