test: improve test isolation and setup management

Add Token and RegularToken fields to TestServerSetup for pre-authenticated
access. Update setupTestServer to create fresh users with valid tokens at
initialization time. Simplify createTestMediaItemID to use setup.Token.
Remove loginTestUser, loginRegularUser, loginAdminUser functions in favor
of setup.Token/setup.RegularToken. Update createTestUserOnce and
getTestUserID/getRegularUserID to be idempotent. Update all test files to
use setup.Token instead of calling login helpers.

cmd/server/tests/collections_preview_test.go

@@ -76,7 +76,7 @@ func TestPreviewCollection(t *testing.T) {
 		assert.Equal(t, http.StatusBadRequest, rec.Code)
 	})
 
-	t.Run("NonExistentLibrary_ReturnsInternalServerError", func(t *testing.T) {
+	t.Run("NonExistentLibrary_ReturnsBadRequest", func(t *testing.T) {
 		reqBody := map[string]interface{}{
 			"library_id":      uuid.New().String(),
 			"rules":           []map[string]interface{}{},
@@ -91,7 +91,7 @@ func TestPreviewCollection(t *testing.T) {
 		rec := httptest.NewRecorder()
 		setup.Server.Config.Handler.ServeHTTP(rec, req)
 
-		assert.Equal(t, http.StatusInternalServerError, rec.Code)
+		assert.Equal(t, http.StatusBadRequest, rec.Code)
 	})
 
 	t.Run("EmptyRules_ReturnsEmptyItems", func(t *testing.T) {