#617 workflow validate: method args with a Zod .default() are treated as required

Description

When a model type's method arguments (or globalArguments) Zod schema declares a field with .default(...), the JSON Schema swamp generates from it still lists that field in required. swamp workflow validate's step-input check (#40) then treats an optional, defaulted argument as mandatory and fails a step that omits it — even though the method runs fine using the default.

This is adjacent to #359 (false "Missing required inputs" when the arg is set in the model definition) but distinct: here the argument is not supplied anywhere — the type schema's .default() should make it optional, and it does not.

Steps to reproduce

1. An extension model type whose method has a defaulted argument:

   methods: {
     audit: {
       arguments: z.object({
         limit: z.number().int().min(1).max(100).default(15),
       }),
       execute: async (_a, ctx) => { /* ... uses args.limit ?? 15 */ },
     },
   }

2. Create an instance (swamp model create <type> m) with methods: {} (the default is defined only on the type's method schema, not in the instance). swamp model type describe <type> --json shows limit in the method arguments' required array despite the .default().

3. Reference the method from a workflow step that does NOT supply limit:

   steps:
     - name: s
       task: { type: model_method, modelIdOrName: m, methodName: audit }

4. swamp workflow validate <wf> --json.

Expected

A Zod field with .default(...) is optional: it should be omitted from the generated JSON Schema required, and swamp workflow validate should pass a step that omits it (the default applies at runtime).

Actual

• Defaulted fields are emitted in required.
• The step-input check fails: Step inputs for '<step>' (m.audit) -> false.
• swamp model method run m audit (no --input) runs fine using the default, confirming the requirement is spurious.

Workaround

Supply the defaulted argument explicitly in the workflow step inputs: (e.g. limit: 15).

Suggested fix

Treat Zod ZodDefault (and ZodOptional) as not-required during the zod -> JSON-Schema conversion; and/or have the step-input check treat a schema default as satisfying the requirement (same spirit as the #359 fix, applied to defaults rather than definition-supplied args).

Environment

Observed on swamp 20260608.234005.0-sha.ed5f78a4 (an update to 20260610.225536.0-sha.4559c368 is available; please verify whether the #359 fix already covers the .default() case). Schemas authored with Zod v4.