MCP tools can have an inputSchema like `{"type": "object", "additionalProperties": false}`
with no `properties` key (valid for zero-arg tools). The hard assert on `"properties" in
parameters_schema` caused an AssertionError. Replace assertions with safe defaults.
Datadog: https://us5.datadoghq.com/error-tracking/issue/ecee9ee2-db75-11f0-ba66-da7ad0900000
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