Support WIT resource types in MCP tool calls

[JMLX42]

Problem

Currently, component2json does not support WIT resource types through MCP:

• Outputs: Resources serialize to debug strings ("resource: {res:?}") — not usable handles
• Inputs: Explicitly rejected with Err(ValError::ResourceError)

This prevents components from exposing interfaces like wasi:filesystem where operations return and accept descriptor resources.

Use Case

We want to build a storage component that exports wasi:filesystem so agents can:

open("/data/file.txt") -> descriptor(3)
read(descriptor(3), 1024) -> bytes
close(descriptor(3))

Without resource support, we must define custom wrapper interfaces that avoid resources entirely — defeating the goal of reusing standard WASI interfaces.

Proposed Solution

Serialize resources as integer handles that remain valid within a session:

Schema generation:

Type::Own(r) | Type::Borrow(r) => json!({
    "type": "integer",
    "description": format!("Handle to resource: {}", resource_name(r))
})

Output conversion:

// When component returns a resource, allocate a handle ID
Val::Resource(res) => {
    let handle_id = resource_table.insert(res);
    json!(handle_id)
}

Input conversion:

// When MCP passes an integer for a resource param, look up the handle
(Type::Own(_) | Type::Borrow(_), Value::Number(n)) => {
    let handle_id = n.as_u64().ok_or(ValError::InvalidHandle)?;
    let res = resource_table.get(handle_id).ok_or(ValError::HandleNotFound)?;
    Ok(Val::Resource(res))
}

Resource Lifecycle

• Handles are scoped to the component instance (each agent session has isolated handles)
• drop resource calls remove from table
• Optional: auto-cleanup on session end

Alternatives Considered

1. Custom wrapper interfaces — Works but forces every component to reimplement path-based wrappers instead of using standard WASI interfaces
2. String-encoded handles — Possible but integers are cleaner and match how file descriptors work

[github-actions]

Agentic Issue Triage triggered by this issue.