Dashboard test surface: invoke an MCP tool by slug (FEATHER_MCP_CLIENT)

Fire a real FEATHER_MCP_CLIENT invocation from the Integrations dashboard.

Verifies the parent connection is owned by the caller's org (404 otherwise), resolves the slug to (connection, tool) via ToolService.resolve_by_slug, then dispatches through our MCP client → the customer's MCP server (decrypt creds → open session → call_tool). This is the only non-agent way to exercise FEATHER_MCP_CLIENT, so a green result here is strong evidence the whole custom-MCP invoke path works end-to-end. Returns the raw MCP result and emits an integration_tool_call Tinybird row with mode=feather_mcp_client / reason="ui_mcp_test_call". An unresolved/disabled slug surfaces as ToolUnavailableError (400).