fix: lazily create async streaming requests - #183

Summary

• defer creation of async streaming request coroutines until the response context manager is entered
• apply the lazy behavior to both standard and custom streamed response wrappers
• add regression coverage for discarded context managers and normal enter/close behavior

Why

Creating an async streaming context manager previously allocated a coroutine immediately. Discarding an unentered manager therefore emitted a RuntimeWarning for an unawaited coroutine.

Fixes #175

Testing

• python -m pytest tests/test_response.py -q -n 0 (30 passed)
• full suite with Steady: 1220 passed, 8 skipped, plus 4 pre-existing Windows/environment failures tracked in Make the test suite deterministic on Windows and proxied environments #181
• full suite excluding those four known failures: 1220 passed, 8 skipped, 4 deselected
• python -m ruff check .
• python -m mypy src/browserbase/_response.py