### What problem does this PR solve?

#9422

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

### What problem does this PR solve?

### Type of change

- [x] Refactoring

### What problem does this PR solve?

Change return type of _generate_streamly from str to Generator[str,
None, None] to properly type hint streaming responses.

### Type of change

- [x] Refactoring

### What problem does this PR solve?


### Type of change

- [x] Refactoring

### What problem does this PR solve?

#9082 #6365

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

### What problem does this PR solve?

#9082 #6365

<u> **WARNING: it's not compatible with the older version of `Agent`
module, which means that `Agent` from older versions can not work
anymore.**</u>

### Type of change

- [x] New Feature (non-breaking change which adds functionality)