Fix an issue with z_filelib:os_filename/1 and embedded single quotes

[mworrell]

This pull request refactors and improves the os_filename/1 function in z_filelib.erl to provide more robust and accurate escaping of filenames for both Unix and Windows systems. It also adds comprehensive unit tests to verify the new behavior. The most important changes are:

Filename escaping improvements

• Refactored the os_filename/1 function to delegate to OS-specific escaping logic, handling Unix and Windows filename escaping separately for improved correctness and maintainability. The function now properly quotes and escapes filenames according to the target OS conventions.
• Added a new internal API, os_filename/2, to allow explicit selection of the target OS family (Unix or Windows) for filename escaping.

Testing

• Added a new test module z_filelib_test.erl with thorough unit tests for both Unix and Windows variants of os_filename/2, covering a variety of edge cases and special characters.

Code organization

• Updated the module export logic to conditionally export os_filename/2 only in test builds, ensuring it is available for testing but not exposed in production.

[Copilot]

Pull request overview

This pull request updates z_filelib:os_filename/1 to perform OS-appropriate filename quoting/escaping (Unix vs. Windows), and adds EUnit coverage to validate edge cases like embedded quotes and trailing backslashes.

Changes:

• Refactors os_filename/1 to delegate to OS-family-specific escaping via a new internal os_filename/2.
• Implements correct Unix single-quote escaping ('\'') and Windows backslash/quote escaping rules inside double quotes.
• Adds a dedicated z_filelib_test.erl EUnit module covering Unix and Win32 cases, including embedded quotes and backslash edge cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/z_filelib.erl	Refactors filename quoting/escaping into OS-specific implementations and adds test-only export for os_filename/2.
test/z_filelib_test.erl	Adds EUnit tests for Unix and Win32 escaping behavior, including embedded quote and trailing backslash scenarios.

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