fix(google_genai): Make sure streaming spans attach to correct parent

[Abhijeet Prasad (AbhiPrasad)]

Implemented a red/green fix for the Google GenAI streaming parent-span issue.

This was noticed when examining a trace w/ livekit agents + vertex ai.

What was happening:

• In trace.json, generate_content_stream LLM spans were parented to the outer/root span instead of the correct parent.
• Root cause is in py/src/braintrust/integrations/google_genai/tracing.py: streaming wrappers created the Braintrust span only when the returned stream was consumed, not when the stream was created.
• Vertex creates the stream while the session parent is active, but consumes it later in a different context, so the parent was lost.

Fix:

• Capture the active Braintrust parent when generate_content_stream / async stream is created.
• Re-apply that captured parent when the stream is later consumed.