refactor(core,logger)migrate to CherryPickObserver API and drop CherryPickLogger

BREAKING CHANGE:
- Removed the deprecated CherryPickLogger interface from cherrypick
- Logger/adapters (e.g., talker_cherrypick_logger) must now implement CherryPickObserver
- talker_cherrypick_logger: replace TalkerCherryPickLogger with TalkerCherryPickObserver
- All usages, docs, tests migrated to observer API
- Improved test mocks and integration tests for observer pattern
- Removed obsolete files: cherrypick/src/logger.dart, talker_cherrypick_logger/src/talker_cherrypick_logger.dart
- Updated README and example usages for new CherryPickObserver model

This refactor introduces a unified observer pattern (CherryPickObserver) to handle all DI lifecycle events, replacing the limited info/warn/error logger pattern.
All external logging adapters and integrations must migrate to use CherryPickObserver.

cherrypick/example/cherrypick_logger_demo.dart

@@ -15,7 +15,7 @@ class AppModule extends Module {
 
 void main() {
   // Set a global logger for the DI system
-  CherryPick.setGlobalLogger(PrintLogger());
+  CherryPick.setGlobalObserver(PrintCherryPickObserver());
 
   // Open the root scope
   final rootScope = CherryPick.openRootScope();
@@ -32,6 +32,6 @@ void main() {
   subScope.closeSubScope('feature.profile');
 
   // Demonstrate disabling and re-enabling logging
-  CherryPick.setGlobalLogger(const SilentLogger());
+  CherryPick.setGlobalObserver(SilentCherryPickObserver());
   rootScope.resolve<UserRepository>(); // now without logs
 }