bug(openapi): empty requestBody emitted for POST routes with path-params-only input

[ghdoergeloh]

Description

When a POST route's input schema contains only path parameters (e.g. z.object({ studyName: z.string() }) for a route /{studyName}/export), the OpenAPI generator emits an empty requestBody:

```yaml
requestBody:
required: false
content:
application/json:
schema:
type: object
properties: {}
required: []
```

This body is meaningless — the endpoint has no request body — and causes noise/confusion in generated clients and documentation.

Why it happens

In packages/openapi/src/openapi-generator.ts, the #inputParams() method (around line 351) calls separateObjectSchema(schema, dynamicParams) to split path params from the rest. When all fields are path params, the remainder is an empty object schema { properties: {}, required: [] }. The generator emits this unconditionally as a requestBody.

Expected behaviour

No requestBody should be emitted when the remainder after path-param separation is an empty object schema.

Suggested fix

Skip emitting requestBody when the remaining schema is an empty object:

```typescript
else {
const isEmptyObject = isObjectSchema(schema)
&& (!schema.properties || Object.keys(schema.properties).length === 0)
&& (!schema.required || schema.required.length === 0)

if (!isEmptyObject) {
ref.requestBody = {
required,
content: toOpenAPIContent(schema),
}
}
}
```

Backward compatibility

Only suppresses requestBody when it would be an empty object. Any route with actual body fields is unaffected.

Reproduction

```typescript
const contract = oc
.route({ method: 'POST', path: '/{studyName}/export' })
.input(z.object({ studyName: z.string() }))
.output(z.string())
```

Discovered while working on #1058.