Implement simplified Antithesis assertions for trace loss tracking

- Add assertions to CoreTracer.write() to track sampling decisions
- Add assertions to RemoteWriter.write() to track buffer drops
- Add assertions to PayloadDispatcherImpl.accept() to track send success/failure

This provides clear tracking of:
- trace_accepted_by_sampling: Traces that passed sampling
- trace_dropped_by_sampling: Traces dropped by sampling
- trace_enqueued_for_send: Traces accepted into buffer
- trace_dropped_buffer_overflow: Traces dropped due to full buffer
- trace_dropped_by_policy: Traces dropped by policy
- trace_dropped_writer_closed: Traces dropped during shutdown
- trace_payloads_being_sent: All send attempts
- traces_sent_successfully: Traces successfully sent to agent
- traces_failed_to_send: Traces that failed HTTP send

Author: robertomonteromiguel

.gitignore

@@ -66,6 +66,9 @@ replay_pid*
 !dd-java-agent/benchmark/releases/*.jar
 **/errors/*.log
 
+# Fuzz testing logs #
+fuzz-logs/
+
 # Magic for local JMC built
 /vendor/jmc-libs