AbstractAgent.onError uses hardcoded console.error, bypassing host application loggers

## Problem

`AbstractAgent.onError` in `sdks/typescript/packages/client/src/agent/agent.ts` (line ~507) calls `console.error` directly:

```ts
console.error('Agent execution failed:', error)
```

This bypasses any structured logging the host application has configured (e.g. Winston, Pino, or other JSON loggers). When the error object contains a stack trace — as `MastraError` does — Node.js serialises it as a multi-line string, producing unstructured, multi-line log entries in log aggregation systems (e.g. Humio, Datadog, Splunk) that expect newline-delimited JSON.

## Impact

- Log aggregation pipelines that parse NDJSON receive broken, multi-line entries for every agent execution failure.
- The error is invisible to the host application's error tracking / alerting stack unless the host also intercepts `console.error` globally.
- There is no way to suppress or redirect these logs without monkey-patching the global `console`.

## Suggested fix

Expose a configurable `logger` option on `AbstractAgent` (or the `RunAgentInput` / constructor options) and use it inside `onError` instead of `console.error`. Falling back to `console.error` when no logger is provided keeps the current behaviour for users who have not opted in.

```ts
// example API
const agent = new MyAgent({ logger: myStructuredLogger });

// inside AbstractAgent.onError
(this.logger ?? console).error('Agent execution failed:', error)
```

Alternatively, an `onError` callback hook would give host applications the same control.

## Environment

- Package: `@ag-ui/client`
- Affected method: `AbstractAgent.onError`
- Node.js serialises objects passed to `console.error` using `util.inspect`, which expands stack traces across multiple lines.