Fix relationship validation to track endpoints for duplicate detection

[Copilot]

Relationship identifiers were incorrectly flagged as duplicates on any reuse. Per spec, relationships can repeat in a path as long as they connect the same nodes.

Valid: (a)-[r]->(b), (a)-[r]->(b)-[r2]->(c) — r connects same endpoints
Invalid: (a)-[r]->(b), (a)-[r]->(c) — r connects different targets

Changes

• Extended PatternSignature with sigEndpoints :: Maybe (Maybe Identifier, Maybe Identifier) to track source/target nodes for relationships
• Updated registerRelationship to compare endpoints when identifier already exists — error only if endpoints differ
• Added registerPathSegments helper to pass node context during path traversal

Tests Added

it "accepts relationship reuse with same endpoints" $
  validateSource "(a)-[r]->(b), (a)-[r]->(b)-[r2]->(c)" `shouldSatisfy` isRight

it "rejects relationship reuse with different endpoints" $
  validateSource "(a)-[r]->(b), (a)-[r]->(c)-[r2]->(b)" `shouldSatisfy` isLeft

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.