Fix UNC path handling during install root path validation - #8258

Describe the pull request

This fixes installer path normalization for repository/log root paths on Windows UNC shares.

Today InstallPost replaces all \\ with / before calling os.MkdirAll. For UNC paths (e.g. \\imb-nas\\repositories) this rewrite can turn a valid Windows network path into an invalid path and trigger:

Repository root path is invalid: mkdir //...: The specified path is invalid.

This PR keeps native Windows paths unchanged while preserving existing normalization behavior on non-Windows platforms.

Link to the issue: #2550

Checklist

• I agree to follow the Code of Conduct by submitting this pull request.
• I have read and acknowledge the Contributing guide.
• I have added test cases to cover the new code or have provided the test plan. (if applicable)
• I have added an entry to CHANGELOG. (if applicable)

Test plan

• Added TestNormalizeInstallRootPathByOS in internal/route/install_test.go to cover:
  • preserving UNC and drive-letter paths on Windows mode
  • preserving existing backslash-to-slash normalization on non-Windows mode
• Verified no whitespace issues with git diff --check.
• Could not run go test in this environment because the Go toolchain is unavailable in the current runner (go: command not found). CI should execute the full test suite.