fix(mcp): preserve operation return docs

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Summary

• pass each registry operation’s full description explicitly to FastMCP
• preserve Returns: contracts on the MCP surface without changing generated input schemas
• add focused coverage comparing the MCP description with the source operation contract

Testing

• uv run --project core ruff format --check .
• uv run --project core ruff check .
• uv run --project core mypy --config-file core/pyproject.toml core/switch_core/ connectors/
• uv run --project core pytest -c core/pyproject.toml core/tests/switch_core/bridges/agent/test_mcp_tool_surface.py core/tests/switch_core/bridges/agent/api/test_operations.py -q (20 passed)

The full non-integration test command reached 2,145 passing tests; 504 database-backed tests could not initialize because no Docker daemon is available in this environment.

Closes #348

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