[opentelemetry-instrumentation-google-genai] Update generate_content streaming variants to use AsyncStreamWrapper and SyncStreamWrapper from utils

[DylanRussell]

Description

Update the generate_content streaming variants to use/return the util streaming wrappers.. This makes the code a bit cleaner and also ensures we let the user invoke the stream when they want to instead of us doing it immediately..

Fixes # (issue)

Type of change

• [x ] Bug fix (non-breaking change which fixes an issue)

How has this been tested?

Unit tests

Checklist

See CONTRIBUTING.md
for the style guide, changelog guidance, and more.

• [x ] Followed the style guidelines of this project
• Changelog updated if the change requires an entry
• [ x] Unit tests added
• [x ] Documentation updated

[Copilot]

Pull request overview

Updates the Google GenAI generate_content_stream (sync + async) instrumentation to return the shared opentelemetry-util-genai stream wrappers, aligning streaming lifecycle finalization with the util’s standardized SyncStreamWrapper/AsyncStreamWrapper behavior.

Changes:

• Introduces GenerateContentStreamWrapper and AsyncGenerateContentStreamWrapper subclasses to drive per-chunk telemetry and finalize spans on stream completion/error.
• Refactors both streaming instrumentations to return the util wrappers instead of hand-rolled generator wrappers.
• Adjusts imports to support the new wrapper types.