Safely escape relation type, fix create and delete relation methods

[EthanKang1]

While testing the mcp-neo4j-memory with Claude, I ran into issues creating and deleting relations with the error

Invalid input '$': expected a node label/relationship type name, '%' or '(' (line 6, column 29 (offset: 204))

meaning $(relation.relationType) was being used directly in the Cypher query rather than substituting the value.

This pull request creates a _safe_relation_type method to sanitize the relationship type, and then uses string interpolation to form the query with the sanitized relation type. This methodology is used for both the create_relations and delete_relations method.