[Feature]: Add messageFilter to AgentConfig

### Pre-flight Checklist

- [x] I have searched existing issues and this hasn't been requested yet.

### Problem or Motivation

When a backend already holds conversation history in memory, there's no way to control which messages get forwarded on each run. The full history is always sent even when only the latest user message is needed.

### Proposed Solution

Add an optional `messageFilter` callback to `AgentConfig`:

```ts
messageFilter?: (messages: Message[]) => Message[]
```

Applied in `prepareRunAgentInput` after stripping `activity` messages, before the input is sent. Stored messages stay untouched. No-op when not set.

### Alternatives Considered

Patching `window.fetch` to strip messages before the request. It works but is hacky and fragile.

### Additional Context

Came up practically in CopilotKit/CopilotKit#1482. I have a working implementation ready.