fix: agent crashes with TypeError when messages is null/undefined

## Bug

`AbstractAgent.messages` is a public property typed as `Message[]`. If it is assigned `null` or `undefined` at runtime, any subsequent call to `runAgent()`, `connectAgent()`, or event processing crashes with:

```
TypeError: Cannot read properties of undefined (reading 'map')
```

or

```
TypeError: Cannot read properties of null (reading 'filter')
```

## Reproduction

```ts
const agent = new MyAgent({ threadId: "t1" });
(agent as any).messages = undefined;
await agent.runAgent(); // TypeError
```

Also reproduces during a live run — if `messages` becomes `undefined` between the time the run starts and when a `RUN_ERROR` event fires, the event-processing pipeline crashes inside the subscriber callback with:

```
TypeError: Cannot read properties of undefined (reading 'map')
at Object.onRunErrorEvent
```

## Root Cause

Seven call sites in `agent.ts` and `default.ts` dereference `this.messages` / `agent.messages` directly without guarding against `null`/`undefined`:

- `runAgent` — `this.messages.map(...)` (build `currentMessageIds` set)
- `connectAgent` — same
- `prepareRunAgentInput` — `structuredClone_(this.messages).filter(...)`
- `runAgent` / `connectAgent` post-run — `structuredClone_(this.messages).filter(...)`
- `onInitialize` — `runSubscribersWithMutation(subscribers, this.messages, ...)`
- `onError` — same
- `defaultApplyEvents` — `structuredClone_(agent.messages)` → passed as `initialMessages` to every subscriber call including `onRunErrorEvent`

## Fix

Add `?? []` fallback at each site so a missing `messages` value is treated as an empty array rather than crashing.