Add conftest fixture to transcribe-streaming integration tests

[Alan4506]

Description of changes:

This PR refactors Transcribe Streaming integration tests, by replacing the standalone setup_resources.py script with a session-scoped pytest fixture in conftest.py. This aligns with the pattern of Lex Runtime V2 integration tests. The HealthScribe test (test_get_medical_scribe_stream) previously required manually running the setup script and exporting environment variables before running tests. Now the IAM role and S3 bucket are automatically created and torn down per test session.

Changes:

• Added conftest.py with healthscribe_resources fixture (creates IAM role + S3 bucket, tears down after)
• Updated test_non_streaming.py to use the fixture instead of environment variables
• Added UUID suffix to role and bucket names to avoid race conditions in parallel runs

To run the integration tests, you should do the following under clients/aws-sdk-transcribe-streaming:

uv venv .venv
source .venv/bin/activate
uv pip install --group test -e . boto3
python -m pytest tests/integration

All tests should pass.

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