Test reveals Guardian config gaps for MCP tools

A researcher tested the OWASP ACS reference Guardian on MCP tools with a script argument and with no arguments. Without a by_tool entry in mapping.yaml, both echo ciao and echo rm -rf / get the same evaluation_failed deny, and a no-argument tool is blocked under every configuration. PR #229 adds a separate tool_unregistered code for tools missing from the manifest.

• Without a by_tool entry, both echo ciao and echo rm -rf / are denied with evaluation_failed
• With mcp__lab2__exec: script, the destructive command is blocked as destructive_shell_command_blocked
• A tool with no arguments is denied under all four Guardian configurations
• PR #229 introduces the tool_unregistered code for tools outside the manifest