Fix | Handle dual-purpose sources with both ref and path in repo-server-api renderer

[dag-andersen]

Summary

Fixes #401

• The repo-server-api renderer failed when a source had both ref and path set simultaneously (a valid Argo CD pattern for cross source references)
• splitSources now treats ref and content (path/chart) as independent concerns - a source with both ends up in both lists

Root Cause

splitSources classified sources into mutually exclusive buckets: a source was either a ref source (ref != "" and path == "") or a content source (everything else). A source with both ref: values and path: manifests was only placed in contentSources, so sibling chart sources couldn't resolve $values/... value files - producing:

source referenced "$values", but no source has a 'ref' field defined

Fix

Replaced the three-way if/else with two independent checks:

if s.Ref != "" {
    refSources = append(refSources, s)
}
if s.Path != "" || s.Chart != "" || s.Ref == "" {
    contentSources = append(contentSources, s)
}

Testing

• Added TestBuildManifestRequest_ExternalChart_WithRefAndPath_GH401 - reproduces the exact scenario from the issue
• All existing unit tests pass (including race detection)