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-flow/trustgraph/agent/react/tools.py

@@ -39,13 +39,14 @@ class KnowledgeQueryImpl:
         if respond:
             from ... schema import AgentResponse
 
-            async def explain_callback(explain_id, explain_graph):
+            async def explain_callback(explain_id, explain_graph, explain_triples=None):
                 self.context.last_sub_explain_uri = explain_id
                 await respond(AgentResponse(
                     chunk_type="explain",
                     content="",
                     explain_id=explain_id,
                     explain_graph=explain_graph,
+                    explain_triples=explain_triples or [],
                 ))
 
         if current_uri: