feat: tighten mcp tool enum parameter typing

[HumanBean17]

Summary

• tighten MCP tool parameter typing with Literal[...] so constrained values (search.table, find.kind, neighbors.direction, neighbors.edge_types[]) reject invalid input at parse time
• introduce shared EdgeType literal alias in mcp_v2.py and wire server.py tool signatures to the same edge-label taxonomy
• add regression coverage for invalid enum-like values and a schema-introspection test that asserts enum emission in tool input schemas

Test plan

• .venv/bin/python -m pytest tests/test_mcp_v2.py -q
• .venv/bin/python -m pytest tests/test_mcp_tools.py -q
• .venv/bin/python -m pytest tests -v
• .venv/bin/ruff check .

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