Clear error_code on entry in win32_unicode_path constructor

[ssam18]

Fixes #3035

Issue

The win32_unicode_path constructor accepts an error_code& parameter but wasn't clearing it on successful path conversion. This led to callers retaining stale error values even when operations succeeded.

Problem

As reported in #3035, when using file.open() with a pre-set error code (e.g., initialized to error::not_found as a default), the error would persist even after successful file operations. This is inconsistent with std::filesystem conventions where error_code parameters are cleared on success.

Solution

Added ec = {}; at the beginning of the constructor to ensure the error_code is always in a known state - either cleared for success or properly set for failure.

This aligns with:

• The std::filesystem API convention (e.g., std::filesystem::create_directory)
• The recommendation from @vinniefalco to clear ec immediately upon entry to avoid these kinds of issues
• Beast's own file_win32::open which already clears ec on line 222

Testing

Windows-specific change. The fix is minimal and follows established error handling patterns in the codebase.

[ssam18]

@vinniefalco - Can you please get a chance to review this PR?

[ashtum]

Please review all file-related operations to ensure they clear ec on success, and also determine how we can add appropriate tests to confirm this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.14%. Comparing base (0620a68) to head (ed41efe).
⚠️ Report is 10 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3056      +/-   ##
===========================================
- Coverage    93.18%   93.14%   -0.04%     
===========================================
  Files          176      177       +1     
  Lines        13703    13706       +3     
===========================================
- Hits         12769    12767       -2     
- Misses         934      939       +5     

Files with missing lines	Coverage Δ
include/boost/beast/core/impl/file_posix.ipp	84.31% <100.00%> (-0.56%)	⬇️
include/boost/beast/core/impl/file_stdio.ipp	85.15% <100.00%> (+0.11%)	⬆️

... and 43 files with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0620a68...ed41efe. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.