ark: partial tools attached to engine-backed agents fail silently

[anuarora1990]

Summary

A partial tool attached to an agent that runs on a named ExecutionEngine is
dropped. As of # the executor log says so clearly, but nothing else
does: no admission warning, and the dashboard cannot see partials at all.
Follow-up to #3247, which covered non-MCP tool types but explicitly left
partials out of scope.

Why partials cannot reach an engine

ToolPartial.Parameters exists to preconfigure and hide parameters — "injected
at runtime and not visible/editable by the agent" (agent_types.go). The only
thing an engine receives is MCPServerConfig:

name, url, transport, timeout, headers, tools