Fix after hook to handle mocked Syslog singleton

The after hook that resets @syslog was causing mock expectation errors
in config_spec tests that stub Syslog.instance to be called exactly
twice. The hook's additional call violated these expectations.

Fix by:
- Wrapping the reset in begin/rescue to catch MockExpectationError
- Only resetting if the instance responds to instance_variable_set
  (protecting against doubles that don't implement this method)

This allows tests that mock the singleton to work correctly while still
preventing mock leakage in tests that use the real singleton.

Author: johha

spec/spec_helper.rb

@@ -207,7 +207,15 @@
 
     rspec_config.after do
       # Reset the Syslog sink singleton to prevent mock leakage between tests
-      Steno::Sink::Syslog.instance.instance_variable_set(:@syslog, nil) if defined?(Steno::Sink::Syslog)
+      # Only reset if it's the actual singleton instance (not a mock/double)
+      if defined?(Steno::Sink::Syslog)
+        begin
+          instance = Steno::Sink::Syslog.instance
+          instance.instance_variable_set(:@syslog, nil) if instance.respond_to?(:instance_variable_set)
+        rescue RSpec::Mocks::MockExpectationError
+          # Ignore mock expectation errors - the test is managing the mock itself
+        end
+      end
     end
 
     rspec_config.after(:each, type: :legacy_api) { add_deprecation_warning }