fix(plugin): install into first path when HELM_PLUGINS has multiple entries

When HELM_PLUGINS contains a colon-separated list of directories,
FindPlugins() correctly reads from all of them using filepath.SplitList().
However, plugin install was ignoring the split and falling back to the
raw HELM_PLUGINS string as the destination directory instead of using
the first entry.

This change adds pluginInstallDir() which returns the first entry from
HELM_PLUGINS when multiple paths are present, consistent with how git
handles multiple path variables and matching user expectations.

Fixes #11310

internal/plugin/installer/base.go

@@ -32,10 +32,25 @@ func newBase(source string) base {
 	settings := cli.New()
 	return base{
 		Source:           source,
-		PluginsDirectory: settings.PluginsDirectory,
+		PluginsDirectory: pluginInstallDir(settings.PluginsDirectory),
 	}
 }
 
+// pluginInstallDir returns the directory to install a new plugin into.
+// When HELM_PLUGINS contains multiple colon-separated paths, the first
+// path is used as the install destination — consistent with how
+// FindPlugins() loads from all paths and matching git conventions.
+func pluginInstallDir(helmPlugins string) string {
+	if helmPlugins == "" {
+		return helmPlugins
+	}
+	dirs := filepath.SplitList(helmPlugins)
+	if len(dirs) == 0 {
+		return helmPlugins
+	}
+	return dirs[0]
+}
+
 // Path is where the plugin will be installed.
 func (b *base) Path() string {
 	if b.Source == "" {

internal/plugin/installer/base_test.go

@@ -44,3 +44,40 @@ func TestPath(t *testing.T) {
 		}
 	}
 }
+
+func TestPluginInstallDir(t *testing.T) {
+	tests := []struct {
+		name        string
+		helmPlugins string
+		want        string
+	}{
+		{
+			name:        "empty string returns empty string",
+			helmPlugins: "",
+			want:        "",
+		},
+		{
+			name:        "single path returned as-is",
+			helmPlugins: "/tmp/plugins",
+			want:        "/tmp/plugins",
+		},
+		{
+			name:        "two paths returns first",
+			helmPlugins: "/tmp/abc:/tmp/xyz",
+			want:        "/tmp/abc",
+		},
+		{
+			name:        "three paths returns first",
+			helmPlugins: "/tmp/a:/tmp/b:/tmp/c",
+			want:        "/tmp/a",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := pluginInstallDir(tt.helmPlugins)
+			if got != tt.want {
+				t.Errorf("pluginInstallDir(%q) = %q, want %q", tt.helmPlugins, got, tt.want)
+			}
+		})
+	}
+}