test(mcp): add comprehensive integration tests for MCP servers (#838)

[devgabriellopes]

Description

Adds comprehensive integration tests for the MCP server implementations (agent_builder_server.py and tools/mcp_server.py). These servers had disproportionately low test coverage (~60-300 LOC of tests for ~3000 LOC of implementation) relative to their complexity and criticality as the main API layer for agent building.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #838

Changes Made

• Added core/tests/test_agent_builder_server_integration.py (~1700 LOC) with tests for:
  • Session lifecycle (create, load, list, delete)
  • Goal management and validation
  • Node operations (add, update, delete, duplicate detection)
  • Edge operations (add, delete, source/target validation)
  • Graph validation (empty graphs, unreachable nodes, context flow)
• Added tools/tests/test_mcp_server_integration.py (~207 LOC) with tests for:
  • Server module imports and structure
  • Tool registration and validation
  • Credential manager initialization
  • Package exports verification
• Used tmp_path fixtures and monkeypatch to mock filesystem operations, avoiding disk artifacts

Testing

Describe the tests you ran to verify your changes:

• Unit tests pass (cd core && pytest tests/)
• Lint passes (cd core && ruff check .)
• Manual testing performed

PYTHONPATH=core:exports python -m pytest core/tests/test_agent_builder_server_integration.py -v
PYTHONPATH=core:exports python -m pytest tools/tests/test_mcp_server_integration.py -v