Refactor graph operation handling and deduplicate Janitor graph tool

[cursor]

Summary

• Centralized graph operation result serialization in a reusable chain-of-responsibility module at src/adapters/graph_operation_result_serializer.py.
• Updated GraphAdapter.execute_operation to consistently serialize backend results via the shared serializer facade.
• Aligned graph client contracts by changing execute_operation return type to Any in src/adapters/interfaces/graph.py and src/lib/neo4j/client.py.
• Removed duplicate Janitor tool implementation by deleting src/core/agents/tools/janitor_agent/JanitorAgentExecuteGraphOperationTool.py and cleaning imports/exports.
• Simplified redundant exception handling in src/core/agents/janitor_agent.py.
• Removed duplicate serialization logic from src/core/agents/tools/kg_agent/KGAgentExecuteGraphOperationTool.py so serialization is handled in one place.

Tests

• Added graph serialization regression tests in tests/test_architecture_refactors.py.
• Added static deduplication checks for Janitor tool imports/exports in tests/test_static_hardening_refactors.py.
• Ran python3 -m unittest tests.test_static_hardening_refactors -v (pass).
• Attempted python3 -m unittest tests.test_architecture_refactors -v but environment is missing runtime deps (pydantic module not installed).

Notes

• Commit: f12f04f
• Branch: cursor/codebase-structural-improvements-baa3