fix: don't rewrite helm chart URLs pointing to a different host (#56)

## Problem
Helm charts like Intel device plugins have download URLs on `github.com` but the chart index is served from `intel.github.io`. The merger rewrites all URLs through the proxy, constructing:
```
https://artifactapi/api/v1/remote/intel-helm/intel/helm-charts/releases/download/...
```
Which proxies to `https://intel.github.io/helm-charts/intel/helm-charts/releases/download/...` — a 404.

## Fix
Compare the download URL host against the remote's base URL host. If they differ, leave the URL as-is so helm downloads directly from the source. Same-host URLs are still rewritten through the proxy.

Also adds `BaseURL` to `MemberIndex` so the merger has the context it needs, and uses the correct `/local/` vs `/remote/` route prefix.

Reviewed-on: #56
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/virtual/engine.go

@@ -79,7 +79,7 @@ func (e *Engine) fetchMemberIndexes(ctx context.Context, virt models.Virtual, pa
 					results[idx] = result{err: fmt.Errorf("local index %q: %w", name, err)}
 					return
 				}
-				results[idx] = result{index: MemberIndex{RemoteName: name, RepoType: remote.RepoType, Body: body}}
+				results[idx] = result{index: MemberIndex{RemoteName: name, RepoType: remote.RepoType, BaseURL: remote.BaseURL, Body: body}}
 				return
 			}
 
@@ -102,7 +102,7 @@ func (e *Engine) fetchMemberIndexes(ctx context.Context, virt models.Virtual, pa
 				return
 			}
 
-			results[idx] = result{index: MemberIndex{RemoteName: name, RepoType: remote.RepoType, Body: body}}
+			results[idx] = result{index: MemberIndex{RemoteName: name, RepoType: remote.RepoType, BaseURL: remote.BaseURL, Body: body}}
 		}(i, memberName)
 	}
 

internal/virtual/helm_merger.go

@@ -54,15 +54,26 @@ func (m *HelmMerger) MergeIndexes(members []MemberIndex, proxyBaseURL string) ([
 				seen[chart][ver.Version] = true
 
 				if proxyBaseURL != "" {
+					routePrefix := "remote"
+					if member.RepoType == "local" {
+						routePrefix = "local"
+					}
+					baseHost := extractHost(member.BaseURL)
+
 					for i, u := range ver.URLs {
 						if strings.HasPrefix(u, "http://") || strings.HasPrefix(u, "https://") {
-							ver.URLs[i] = fmt.Sprintf("%s/api/v1/remote/%s/%s",
+							if baseHost != "" && extractHost(u) != baseHost {
+								continue
+							}
+							ver.URLs[i] = fmt.Sprintf("%s/api/v1/%s/%s/%s",
 								strings.TrimRight(proxyBaseURL, "/"),
+								routePrefix,
 								member.RemoteName,
 								extractPath(u))
 						} else {
-							ver.URLs[i] = fmt.Sprintf("%s/api/v1/remote/%s/%s",
+							ver.URLs[i] = fmt.Sprintf("%s/api/v1/%s/%s/%s",
 								strings.TrimRight(proxyBaseURL, "/"),
+								routePrefix,
 								member.RemoteName,
 								u)
 						}
@@ -78,6 +89,19 @@ func (m *HelmMerger) MergeIndexes(members []MemberIndex, proxyBaseURL string) ([
 	return yaml.Marshal(merged)
 }
 
+func extractHost(rawURL string) string {
+	idx := strings.Index(rawURL, "://")
+	if idx == -1 {
+		return ""
+	}
+	rest := rawURL[idx+3:]
+	slashIdx := strings.Index(rest, "/")
+	if slashIdx == -1 {
+		return rest
+	}
+	return rest[:slashIdx]
+}
+
 func extractPath(rawURL string) string {
 	idx := strings.Index(rawURL, "://")
 	if idx == -1 {

internal/virtual/merger.go

@@ -9,6 +9,7 @@ import (
 type MemberIndex struct {
 	RemoteName string
 	RepoType   models.RepoType
+	BaseURL    string
 	Body       []byte
 }