Add Polly client integration tests

[jonathan343]

Summary

Adds integration tests for the aws-sdk-polly client covering the three operation shapes the service exposes:

• Non-streaming describe_voices
• Output-streamingsynthesize_speech
• Bidirectional event stream start_speech_synthesis_stream

Each shape exercises a distinct code path in the generated client. Tests are configured against us-east-1 with the generative engine on the Matthew voice and rely on EnvironmentCredentialsResolver, matching the credential pattern used elsewhere in this repo.

Testing

• Set credentials in my environment
• cd clients/aws-sdk-polly
• uv venv && . .venv/bin/activate
• uv pip install -e . --group test

Ran Tests:

(aws-sdk-polly) $ python -m pytest tests/integration
===================================================== test session starts =====================================================
platform darwin -- Python 3.14.5, pytest-9.0.3, pluggy-1.6.0
rootdir: /Users/gytndd/dev/GitHub/aws-sdk-python/clients/aws-sdk-polly
configfile: pyproject.toml
plugins: asyncio-1.3.0
asyncio: mode=Mode.AUTO, debug=False, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collected 3 items

tests/integration/test_bidirectional_streaming.py .                                                                     [ 33%]
tests/integration/test_non_streaming.py .                                                                               [ 66%]
tests/integration/test_output_streaming.py .                                                                            [100%]

====================================================== warnings summary =======================================================
tests/integration/test_non_streaming.py::test_describe_voices
tests/integration/test_output_streaming.py::test_synthesize_speech
  /Users/gytndd/dev/GitHub/aws-sdk-python/clients/aws-sdk-polly/.venv/lib/python3.14/site-packages/aws_sdk_signers/signers.py:794: AWSSDKWarning: Payload signing is enabled. This may result in decreased performance for large request bodies.
    warnings.warn(

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
================================================ 3 passed, 2 warnings in 2.74s ================================================

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