Define Declarative WebMCP integration with form-associated custom elements

[beaufortfrancois]

We need to define how declarative WebMCP handles form-associated custom elements:

• Do we introduce a new setToolSchema() on ElementInternals to set their input schema?
• How can toolparamtitle and toolparamdescription be used?
• How do we notify the custom element when data needs to be filled due to a tool execution? What happens it it takes too long?
• Do they also need a name to be valid?