Remove blocking config portal mode and refactor API

- Remove setConfigPortalBlocking() API and _configPortalIsBlocking member variable
- Remove blocking while(1) loop from startConfigPortal()
- Change startConfigPortal() return type from boolean to void
- Change stopConfigPortal() return type from bool to void
- Make stopConfigPortal() immediately shutdown portal (call shutdownConfigPortal() directly)
- Remove unused portalAbortResult and portalTimeoutResult variables
- Update all comments to remove blocking mode references
- Simplify debug messages (remove 'non-blocking' references)
- Update tests to remove setConfigPortalBlocking() calls
- All 43 tests passing

The config portal is now always non-blocking, requiring users to call
process() periodically in their main loop. This aligns with the async
web server architecture and simplifies the API.

test/test_wifimanager/tests/test_configuration.cpp

@@ -36,8 +36,6 @@ void test_configuration_setters() {
     wm.setShowDnsFields(false);
     
     // Test portal behavior
-    wm.setConfigPortalBlocking(true);
-    wm.setConfigPortalBlocking(false);
     wm.setCaptivePortalEnable(true);
     wm.setCaptivePortalEnable(false);