test(handlers): add Uninstall 503 coverage for plugins_install.go

Adds TestPluginUninstall_ContainerNotRunning_Returns503 which exercises
the 503 path where neither a local Docker container nor a SaaS
instance-id dispatch resolves. The handler must return "workspace
container not running" — not a generic 500 or a misleading 422.

All other paths in plugins_install.go (Install, Download, allowlist,
external-runtime guard) are already covered by the existing test suite
in plugins_test.go, plugins_install_external_test.go, and
org_plugin_allowlist_test.go.

Closes #1377

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workspace-server/internal/handlers/plugins_install_test.go

@@ -0,0 +1,53 @@
+package handlers
+
+// plugins_install_test.go — additional coverage for plugins_install.go.
+//
+// Gaps filled vs. existing test files:
+//   - plugins_install_external_test.go: Install + Uninstall 422 (external runtime) ✓ covered
+//   - plugins_test.go:                 Install 400 (missing source, invalid body, etc.) ✓ covered
+//                                    Uninstall 400 (invalid plugin name, empty name) ✓ covered
+//                                    Download auth gate ✓ covered
+//   - org_import_helpers_test.go:      countWorkspaces, envRequirementKey, sanitizeEnvMembers,
+//                                        flattenAndSortRequirements, collectOrgEnv ✓ covered
+//
+// New test added here:
+//   - Uninstall 503: container not running, no SaaS dispatch.
+//
+// NOTE: validateWorkspaceID is not called inside the Install/Uninstall handlers.
+// UUID validation is the responsibility of the WorkspaceAuth middleware, so no
+// 400 test is needed here for UUID format.
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+)
+
+// TestPluginUninstall_ContainerNotRunning_Returns503 exercises the 503 path
+// where neither a local Docker container nor a SaaS instance-id dispatch
+// resolves. The handler must return "workspace container not running" — NOT a
+// generic 500 or a misleading 422 (external-runtime) message.
+func TestPluginUninstall_ContainerNotRunning_Returns503(t *testing.T) {
+	// No docker client + no instance-id lookup → falls through to 503.
+	h := NewPluginsHandler(t.TempDir(), nil, nil)
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{
+		{Key: "id", Value: "550e8400-e29b-41d4-a716-446655440000"},
+		{Key: "name", Value: "some-plugin"},
+	}
+	c.Request = httptest.NewRequest("DELETE",
+		"/workspaces/550e8400-e29b-41d4-a716-446655440000/plugins/some-plugin", nil)
+
+	h.Uninstall(c)
+
+	require.Equal(t, http.StatusServiceUnavailable, w.Code)
+	var body map[string]string
+	json.Unmarshal(w.Body.Bytes(), &body)
+	require.Equal(t, "workspace container not running", body["error"])
+}