feat(ai): implement experimental_prepareStep for dynamic step configuration

- Add PrepareStepFunction and PrepareStepResult types for step-by-step customization
- Enable dynamic model, toolChoice, and activeTools modification per step
- Support system prompt and messages override during multi-step execution
- Implement experimental_prepareStep parameter in generateText and streamText
- Add comprehensive test coverage for experimental_prepareStep functionality
- Fix stepNumber calculation bug in stream-text.ts (use currentStep instead of recordedSteps.length)

Author: yaonyan