outputFormat returns subtype 'success' when structured_output is absent — should return error_max_structured_output_retries

## Bug: `outputFormat` returns `subtype: "success"` with no `structured_output` for non-trivial schemas

### Environment
- SDK version: `0.2.97`
- Node.js: v22
- OS: Windows 11
- Models tested: `claude-sonnet-4-6`, `claude-haiku-4-5`

### Description

When using `outputFormat` with `query()`, the SDK returns `subtype: "success"` but `structured_output` is `undefined` (key absent from result message) for schemas beyond trivial complexity. The model produces correct JSON in `result` (wrapped in markdown), but the SDK does not populate `structured_output` or return `error_max_structured_output_retries`.

### Reproduction

**Simple schema — WORKS:**
```typescript
for await (const msg of query({
prompt: 'Return JSON with greeting "hello" and number 42.',
options: {
model: "claude-sonnet-4-6",
outputFormat: {
type: "json_schema",
schema: {
type: "object",
properties: {
greeting: { type: "string" },
number: { type: "integer" },
},
required: ["greeting", "number"],
additionalProperties: false,
},
},
maxTurns: 3,
permissionMode: "bypassPermissions",
},
})) {
if (msg.type === "result") {
console.log(msg.subtype); // "success"
console.log(msg.structured_output); // { greeting: "hello", number: 42 } ✅
}
}
```

**Nested schema — FAILS (no `structured_output`):**
```typescript
for await (const msg of query({
prompt: 'Return JSON with version "1", metadata {count:2, label:"test"}, items [{id:"A1", title:"First", status:"active", tags:["a"]}].',
options: {
model: "claude-sonnet-4-6",
outputFormat: {
type: "json_schema",
schema: {
type: "object",
properties: {
version: { type: "string" },
metadata: {
type: "object",
properties: {
count: { type: "integer" },
label: { type: "string" },
},
required: ["count", "label"],
additionalProperties: false,
},
items: {
type: "array",
items: {
type: "object",
properties: {
id: { type: "string" },
title: { type: "string" },
status: { type: "string", enum: ["active", "inactive"] },
tags: { type: "array", items: { type: "string" } },
},
required: ["id", "title", "status", "tags"],
additionalProperties: false,
},
},
},
required: ["version", "metadata", "items"],
additionalProperties: false,
},
},
maxTurns: 3,
permissionMode: "bypassPermissions",
},
})) {
if (msg.type === "result") {
console.log(msg.subtype); // "success" (should be error_max_structured_output_retries?)
console.log(msg.structured_output); // undefined ❌
console.log(msg.result); // "Here is the JSON: ```json\n{...}\n```" (correct JSON in text!)
}
}
```

### What we tested

| Schema | `$ref`/`definitions` | `additionalProperties: false` | `structured_output` |
|---|---|---|---|
| Simple (2 fields) | No | Root only | ✅ Present |
| Nested (3 levels, no `$ref`) | No | Every object | ❌ Absent |
| Deep with `$ref` | Yes | Root only | ❌ Absent |
| Inlined (resolved `$ref`) | No | Every object | ❌ Absent |

All failing cases return `subtype: "success"` — never `error_max_structured_output_retries`.

### Expected behavior

1. If the schema is supported: `structured_output` should be populated with validated JSON
2. If the schema is too complex: `subtype` should be `error_max_structured_output_retries`, not `success`
3. The SDK should never return `subtype: "success"` with `structured_output: undefined` when `outputFormat` is configured

### Notes

- The raw Messages API (`tool_use` + `tool_choice`) correctly handles these schemas — the API enforces nested types including enums and `$ref`. This is an SDK-specific issue.
- The model consistently produces correct JSON in `result` text (wrapped in markdown), showing it understands the schema — the SDK just doesn't extract/validate it.
- Per the [structured outputs docs](https://code.claude.com/docs/en/agent-sdk/structured-outputs), `outputFormat` should work with multi-turn tool use: "The agent can use any tools it needs to complete the task, and you still get validated JSON matching your schema at the end."