What this PR does / why we need it:

HELM_PLUGINS can list several directories separated by the OS path-list separator. Post-renderer lookup (filepath.SplitList in pkg/postrenderer) and helm plugin list handle that, but collectGetterPlugins passes the whole string to plugin.FindPlugins as one directory:

plgs, err := plugin.FindPlugins([]string{settings.PluginsDirectory}, d)

With more than one entry no getter plugin is found, so every plugin-backed scheme fails (helm pull s3://…, git+https://…, etc.), even though helm plugin list shows the plugin. Helm 3 split the list here (FindPlugins iterated filepath.SplitList(plugdirs)), so this is a v4 regression. The fix is filepath.SplitList(settings.PluginsDirectory).

The same file also had fmt.Errorf("plugin %q failed to invoke: %w", g.plg, err), which formats the entire plugin struct. It now uses g.plg.Metadata().Name.

Special notes for your reviewer:

End to end, with helm built from main and from this branch. A getter/v1 subprocess plugin for gettest:// sits in the second of two HELM_PLUGINS entries:

$ HELM_PLUGINS=/tmp/x/empty:/tmp/x/plugins helm pull gettest://charts/demo-0.1.0.tgz
main:         Error: scheme "gettest" not supported
this branch:  (downloads demo-0.1.0.tgz)

When the getter exits non-zero:

main:         Error: plugin &{{"v1" "gettest" "getter/v1" "subprocess" "0.1.0" "" %!q(*schema.ConfigGetterV1=&{[gettest]}) %!q(*plugin.RuntimeConfigSubprocess=…
this branch:  Error: plugin "gettest" failed to invoke: plugin "gettest" exited with error

#32666 rewrites the same FindPlugins line (to envVars["HELM_PLUGINS"]) and also passes a single directory. Whichever lands second only needs filepath.SplitList(...) around it.

If applicable:

• this PR contains user facing changes (the docs needed label should be applied if so)
• this PR contains unit tests
• this PR has been tested for backwards compatibility

Tests: TestCollectPluginsMultiplePluginDirs (on main: "[]" should have 2 item(s), but has 0) and TestGetterPluginInvokeErrorNamesPlugin (on main: the struct dump above) both pass with the change. A single-directory HELM_PLUGINS behaves exactly as before. go test ./pkg/getter/, go vet and golangci-lint v2.14.0 are clean on Go 1.26.