fix(google_genai): Make sure streaming spans attach to correct parent (#484)

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.

Author: AbhiPrasad

py/src/braintrust/integrations/google_genai/cassettes/1.30.0/test_async_stream_completion_preserves_creation_parent_when_consumed_later.yaml

@@ -0,0 +1,68 @@
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py/src/braintrust/integrations/google_genai/cassettes/1.30.0/test_async_stream_completion_preserves_no_parent_when_consumed_under_parent.yaml

@@ -0,0 +1,68 @@
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py/src/braintrust/integrations/google_genai/cassettes/1.30.0/test_stream_completion_preserves_creation_parent_when_consumed_later.yaml

@@ -0,0 +1,68 @@
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py/src/braintrust/integrations/google_genai/cassettes/1.30.0/test_stream_completion_preserves_no_parent_when_consumed_under_parent.yaml

@@ -0,0 +1,68 @@
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py/src/braintrust/integrations/google_genai/cassettes/1.75.0/test_async_stream_completion_preserves_creation_parent_when_consumed_later.yaml

@@ -0,0 +1,68 @@
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