feat: accumulate streaming continuation usage metadata and expose initial_prompt_token_count

[copybara-service]

feat: accumulate streaming continuation usage metadata and expose initial_prompt_token_count

[adri22235]

Hi! Just noticed the CI Run mypy (3.11) check is failing due to two attribute typing errors.

Here are the two minimal fixes that cleanly resolve the Mypy check (Success: no issues found in 41 source files):

1. In google/genai/_extra_utils.py:

# TypeVar "C" does not guarantee the attribute, use getattr or type ignore:
else getattr(curr, "initial_prompt_token_count", None)  # type: ignore[attr-defined]
In google/genai/models.py:
code
Python
# Mypy requires type ignore for async for over an evaluated awaitable:
async for chunk in await self._generate_content_stream(  # type: ignore[attr-defined]
(Optional note: In google/genai/_local_tokenizer_loader.py:83, changing return cast(bytes, resp.content) to return resp.content also clears the [redundant-cast] warning).
We verified these fixes on our CI pipeline and all typing/import jobs pass completely in green. Hope this helps get the PR unblocked!