feat: Add CheckpointService for agent state persistence

Add stateless CheckpointService using SessionService and ArtifactService
primitives with delta compression, concurrent session locking, and
configurable retention. Includes CheckpointableMixin for any agent type,
GraphCheckpointCallback for node-level checkpointing, and comprehensive
error model with telemetry. Includes checkpoint samples.

Author: drahnreb

contributing/samples/graph_agent_advanced/README.md

@@ -0,0 +1,10 @@
+"""Advanced GraphAgent example showcasing all features.
+
+This example demonstrates:
+- Checkpointing with resume capability
+- LLM-based interrupt reasoning
+- Callback-based observability
+- Flexible interrupt timings (BEFORE/AFTER/BOTH)
+- Immediate cancellation with state preservation
+- All interrupt actions (continue, pause, defer, rerun, skip)
+"""