[Bug] streamEvents() throws NPE when generateOptions is not set on HarnessAgent

## Bug Report

**Version:** `agentscope-harness:2.0.0-RC1`
**Java:** 21
**Model backend:** Ollama (qwen3:8b)

---

### Description

When calling `HarnessAgent.streamEvents()` without explicitly setting `generateOptions` on the builder, a `NullPointerException` is thrown because the internal options field is `null`.

```
Cannot invoke "io.agentscope.core.model.GenerateOptions.getToolChoice()"
because "options" is null
```

`HarnessAgent.call()` (non-streaming) works correctly under the same configuration.
Only `streamEvents()` is affected.

---

### Steps to Reproduce

```java
HarnessAgent agent = HarnessAgent.builder()
.name("my-agent")
.sysPrompt("You are a helpful assistant.")
.model("ollama:qwen3:8b")
.workspace(Paths.get(".agentscope/workspace"))
// ← generateOptions NOT set
.build();

// This throws NPE ↓
agent.streamEvents(new UserMessage("Hello"), ctx)
.filter(e -> e.getType() == AgentEventType.TEXT_BLOCK_DELTA)
.cast(TextBlockDeltaEvent.class)
.map(TextBlockDeltaEvent::getDelta)
.subscribe(System.out::print);
```

---

### Workaround

Explicitly passing an empty `GenerateOptions` fixes the issue:

```java
HarnessAgent agent = HarnessAgent.builder()
.name("my-agent")
.sysPrompt("You are a helpful assistant.")
.model("ollama:qwen3:8b")
.workspace(Paths.get(".agentscope/workspace"))
.generateOptions(GenerateOptions.builder().build()) // ← add this
.build();
```

---

### Root Cause (suspected)

Inside `streamEvents` → `delegate.streamEvents`, the code calls `options.getToolChoice()` without a null-check on `options`. When `generateOptions` is not provided via the builder, `options` remains `null`.

**Suggested fix:** Either initialize `generateOptions` to `GenerateOptions.builder().build()` by default in the builder, or add a null-check before accessing `options.getToolChoice()`.

---

### Environment

| Item | Value |
|------|-------|
| OS | macOS |
| Java | 21 |
| Spring Boot | 3.3.0 |
| agentscope-harness | 2.0.0-RC1 |
| Model | Ollama / qwen3:8b |