fix(core): default approval behavior for mcp missing annotations

[fouad-openai]

• Changed requires_mcp_tool_approval to apply MCP spec defaults when annotations are missing.
• Unannotated tools now default to:
  • readOnlyHint = false
  • destructiveHint = true
  • openWorldHint = true
• This means unannotated MCP tools now go through approval/ARC monitoring instead of silently bypassing it.
• Explicitly read-only tools still skip approval unless they are also explicitly marked destructive.

Previous behavior
Failed open for missing annotations, which was unsafe for custom MCP tools that omitted or forgot annotations.