Include accumulated text in OnTextEnd callback

Currently `OnTextEndFunc` receives only the message ID:

```go
OnTextEndFunc func(id string) error
```

Consumers who need the full accumulated text (e.g., to collect or persist completed messages) must buffer all deltas themselves. This duplicates the internal `activeTextContent` map already maintained by the agent.

`OnReasoningEndFunc` already passes the full content:

```go
OnReasoningEndFunc func(id string, reasoning ReasoningContent) error
```

Proposal: add the accumulated text as a second parameter:

```go
OnTextEndFunc func(id string, text string) error
```

The text is already available at the call site from `activeTextContent[part.ID]`.