fix: URL-encode in Vault secret paths

[monchaio]

Summary

• Fix bug where Vault secret paths containing backslashes were not properly handled
• Backslashes are now URL-encoded to prevent path parsing errors

Problem

• Secret paths with backslashes (e.g., domain\user style paths) were failing to sync because the backslash was interpreted as a path separator instead of part of the secret name.

Test Plan

• Added unit tests for backslash encoding
• Verified syncing secrets with \ characters in the path

[pbchekin]

Thank you for identifying the issue and for the fix. To go forward, I think there should be an integration test added to the code. The issue itself needs to be fixed in the underlying low level library vault-rs, which is forked and vendored in this repository. Do you think you can make the required changes?