Fix: Handle OCI artifact versions with '+' correctly

This change addresses an issue where Helm's automatic replacement of '+'
with '_' in version strings causes problems with OCI artifact registries
that do not support '+' in tags.

The following improvements have been made:

- Modified `internal/helper/helper.go`:
    - Added comprehensive documentation to the `SemVerReplace` function.
    - Clarified that `SemVerReplace` focuses on underscore-to-plus
      conversion and does not perform full semver validation.
    - Simplified the `SemVerReplace` function logic.
    - Added extensive unit tests for `SemVerReplace` covering various
      scenarios, including empty strings, multiple underscores,
      leading/trailing underscores, and mixed-case strings.

- Updated `internal/manifest/charts.go`:
    - Added debug logging before attempting underscore-to-plus conversion
      for chart versions.
    - Enhanced error messages to be more specific when a chart version is
      not found, indicating that lookups were attempted with both the
      original reference and the converted reference.

- Updated `internal/manifest/manifest.go`:
    - Ensured consistent handling of underscore-to-plus conversion for
      manifest targets in both GET and HEAD request methods.
    - Added debug logging before attempting underscore-to-plus conversion
      for manifest targets in both GET and HEAD methods.
    - Enhanced error messages to be more specific when a manifest is not
      found, indicating that lookups were attempted with both the
      original target and the converted target.

These changes ensure that versions containing '+' are correctly processed
by first attempting the lookup with the original reference (which might
contain '_') and then, if that fails, by attempting the lookup again
after converting underscores to plus signs. This provides more robust
handling for OCI artifacts and improves debuggability.