Fix: add support for explicit textInput in Pipeline execution missing for Gemini

[techmahedy]

Previously, the execute method relied strictly on the internal $this->messages state.

public function execute(array $params, bool $complete = false, ?string $textInput = null): mixed
{
    $result = $this->platform->invoke($this->model, $textInput ?? $this->messages, $params);

    return $complete ? $result : $result->asText();
}

previously it was

 $result = $this->platform->invoke($this->model, $this->messages, $params);

now we can use

// Old way: Relied on internal message state
$pipeline->execute($params);

// New way: Direct input for one-off tasks
$result = $pipeline->execute(
    params: $params, 
    textInput: 'Hello, how are you?'
);