Use named constant for select timeout instead of magic number

Co-authored-by: bbockelm <1093447+bbockelm@users.noreply.github.com>

Author: Copilot

src/scitokens_internal.h

@@ -469,6 +469,9 @@ class Validator {
     Validator() : m_now(std::chrono::system_clock::now()) {}
 
     void set_now(std::chrono::system_clock::time_point now) { m_now = now; }
+    
+    // Maximum timeout for select() in microseconds for periodic checks
+    static constexpr long MAX_SELECT_TIMEOUT_US = 50000; // 50ms
 
     std::unique_ptr<AsyncStatus> verify_async(const SciToken &scitoken) {
         const jwt::decoded_jwt<jwt::traits::kazuho_picojson> *jwt_decoded =
@@ -496,10 +499,10 @@ class Validator {
 
             while (!result->m_done) {
                 auto timeout_val = result->get_timeout_val(expiry_time);
-                // Limit select to 50ms for periodic checks
-                if (timeout_val.tv_sec > 0 || timeout_val.tv_usec > 50000) {
+                // Limit select to MAX_SELECT_TIMEOUT_US for periodic checks
+                if (timeout_val.tv_sec > 0 || timeout_val.tv_usec > MAX_SELECT_TIMEOUT_US) {
                     timeout_val.tv_sec = 0;
-                    timeout_val.tv_usec = 50000;
+                    timeout_val.tv_usec = MAX_SELECT_TIMEOUT_US;
                 }
 
                 select(result->get_max_fd() + 1, result->get_read_fd_set(),