Fix: Propagate Generic Type to GenerateStreamResponse.stream

[Dinesh-Gautam]

Fixes: #3603

Context

The GenerateStreamResponse interface did not propagate its generic O type to the stream property.
As a result, the chunks yielded by stream were always typed as GenerateResponseChunk<unknown>,
forcing users to manually assert types and losing schema-based type inference.

What Was Changed

Updated the GenerateStreamResponse interface to propagate the generic O type to the stream property.

After Changes

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)