genai-util: refactor streams to a generic ABC wrapper. (#4500)

* wip: move the streams to a generic ABC wrapper.

* polish: add changelogs.

* wip: precommit.

* wip: fix the merge issues.

* polish: pr feedback.

* polish: fixing the precommit.

* wip: renamed to stream.py and changed finish_reasons population style.

* wip: implementing copilot suggestions.

* wip: fixing the tox.

* polish: copilot feedback.

* polish: copilot suggestions.

* Update CLAUDE.md

Co-authored-by: Liudmila Molkova <neskazu@gmail.com>

* wip: shifting the instructions to right agents.md file.

* wip: using objectproxy.

* fixing pypy errors.

* wip: renaming functions based on events.

* polish: making chat_buffers and chat_wrappers type safe.

* polish: removed get_attr guard.

* polish: add types to chat_wrappers.py.

* wip: use get_attr for new fields to avoid failures.

* polish: fix lint and spdx headers.

* polish:fixing pre-commit

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Co-authored-by: Leighton Chen <lechen@microsoft.com>
Co-authored-by: Liudmila Molkova <neskazu@gmail.com>
Co-authored-by: Aaron Abbott <aaronabbott@google.com>