Forward missing explain_triples through RAG clients and agent tool callback (#768)

fix: forward explain_triples through RAG clients and agent tool callback
- RAG clients and the KnowledgeQueryImpl tool callback were
  dropping explain_triples from explain events, losing provenance
  data (including focus edge selections) when graph-rag is invoked
  via the agent.

Tests for provenance and explainability (56 new):
- Client-level forwarding of explain_triples
- Graph-RAG structural chain
  (question → grounding → exploration → focus → synthesis)
- Graph-RAG integration with mocked subsidiary clients
- Document-RAG integration
  (question → grounding → exploration → synthesis)
- Agent-orchestrator all 3 patterns: react, plan-then-execute,
  supervisor

trustgraph-base/trustgraph/base/graph_rag_client.py

@@ -15,7 +15,7 @@ class GraphRagClient(RequestResponse):
             user: User identifier
             collection: Collection identifier
             chunk_callback: Optional async callback(text, end_of_stream) for text chunks
-            explain_callback: Optional async callback(explain_id, explain_graph) for explain notifications
+            explain_callback: Optional async callback(explain_id, explain_graph, explain_triples) for explain notifications
             timeout: Request timeout in seconds
 
         Returns:
@@ -30,7 +30,7 @@ class GraphRagClient(RequestResponse):
             # Handle explain notifications
             if resp.message_type == 'explain':
                 if explain_callback and resp.explain_id:
-                    await explain_callback(resp.explain_id, resp.explain_graph)
+                    await explain_callback(resp.explain_id, resp.explain_graph, resp.explain_triples)
                 return False  # Continue receiving
 
             # Handle text chunks