tool.onInputAvailable can be called without onInputStart being called

### Description

I noticed that the AI SDK can call `onInputAvailable` without ever calling `onInputStart`; forcing the logic to take this into consideration.

I'm wondering if `onInputStart` should not be called in ALL cases (even if the input is available in one go).

### AI SDK Version

- ai: 5.0.107

### Code of Conduct

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