Fix onFinal callback argument when a string is returned from tool call.

[JanMatas]

There is an inconsistent behaviour of onFinal callback when different types are returned from experimental_onToolCall.

When experimental_onToolCall callback returns a stream, onFinal gets called with a complete response of the returned stream. However, when experimental_onToolCall returns a string, onFinal callback is called with the tool call instead of the returned string.

This PR makes the behaviour consistent - onFinal is called with the string returned from experimental_onToolCall.

[lgrammel]

Looks good. I'll add a test case once it's landed.