fix: resolve KnowledgeGraphNode hash collision due to template rendering bugs

Two bugs in KnowledgeGraphNode caused hash/get_content to ignore actual
entity and relationship data, producing identical outputs for completely
different knowledge graph nodes.

Bug 1: Templates used Jinja2 double-brace syntax ({{ name }}) but were
rendered with Python str.format(), which treats {{ as literal {. All
format kwargs were silently ignored.

Bug 2: _get_relationships_str() used self.entity_template instead of
self.relationship_template, so relationship data never reached the output.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 79475432@qq.com

backend/app/rag/retrievers/knowledge_graph/schema.py

@@ -197,14 +197,14 @@ def retrieve_knowledge_graph(self, query_str: str) -> KnowledgeGraphRetrievalRes
 {relationships_str}
 """
 DEFAULT_ENTITY_TMPL = """
-- Name: {{ name }}
-  Description: {{ description }}
+- Name: {name}
+  Description: {description}
 """
 DEFAULT_RELATIONSHIP_TMPL = """
-- Description: {{ rag_description }}
-  Weight: {{ weight }}
-  Last Modified At: {{ last_modified_at }}
-  Meta: {{ meta }}
+- Description: {rag_description}
+  Weight: {weight}
+  Last Modified At: {last_modified_at}
+  Meta: {meta}
 """
 
 
@@ -283,7 +283,7 @@ def _get_relationships_str(self) -> str:
         strs = []
         for relationship in self.relationships:
             strs.append(
-                self.entity_template.format(
+                self.relationship_template.format(
                     rag_description=relationship.rag_description,
                     weight=relationship.weight,
                     last_modified_at=relationship.last_modified_at,