Add missing MCP tool annotations (destructiveHint, idempotentHint, openWorldHint) to ToolAnnotations #119

The current WebMCP spec only defines one tool annotation:

dictionary ToolAnnotations {
boolean readOnlyHint; ← only this exists
};

But MCP (which WebMCP is explicitly designed to align with) defines four annotations. Three are completely missing from WebMCP.

• destructiveHint
• idempotentHint
• openWorldHint

1. destructiveHint — Without it, the browser cannot tell the difference between a safe tool and a catastrophic one (e.g. editDesign() vs deleteAccount()). It has no basis to automatically protect the user with a confirmation step.

2. idempotentHint — Without it, when a tab freezes or times out mid tool call, the agent cannot safely decide whether to retry. It risks either never completing the action or executing it twice (e.g. placing two orders).

3. openWorldHint — Without it, the browser cannot tell whether a tool sends user data to external services. It cannot warn the user or treat results from external sources with appropriate suspicion.