fix: avoid blocking AsyncConversation startup

[ogarciarevett]

Summary

AsyncConversation.start_session() currently performs the authenticated signed-URL request synchronously on the event-loop thread. Run that existing synchronous helper in the loop executor so authenticated startup does not pause unrelated async work.

Closes #811.

Tests

• Added a regression test that verifies signed-URL resolution runs outside the event-loop thread.
• python -m pytest tests/test_async_convai.py -q on Python 3.8.20 — 10 passed
• python tests/verify_types.py — 1,570 type files passed
• mypy . — 2,246 source files passed
• ruff check tests/test_async_convai.py

[PaulAsjes]

Thanks!