Reset codec in syslog singleton to prevent mock leakage

The syslog singleton also stores @codec which can be mocked in tests.
Previously we only reset syslog but not codec, causing mock leakage
into other tests when the codec double was accessed.

Changes:
- Reset both syslog and codec in the after hook
- Add nil guard for codec in add_record method
- Prevents "double leaked into another example" errors

Author: johha

lib/steno/sink/syslog.rb

@@ -36,6 +36,7 @@ def open(identity)
   def add_record(record)
     return if record.log_level == :off
     return unless @syslog
+    return unless @codec
 
     record = truncate_record(record)
     msg = @codec.encode_record(record)

spec/spec_helper.rb

@@ -215,6 +215,7 @@
         begin
           instance = Steno::Sink::Syslog.instance
           instance.instance_variable_set(:@syslog, nil) if instance.respond_to?(:instance_variable_set)
+          instance.instance_variable_set(:@codec, nil) if instance.respond_to?(:instance_variable_set)
         rescue StandardError
           # Ignore any errors - the test may be managing the singleton itself
         end