fix: replace nest_asyncio for Python 3.14 compatibility

[lior-airis]

Summary

• nest_asyncio is unmaintained and incompatible with Python 3.14
• On Python 3.14, asyncio.current_task() returns None under nest_asyncio's patched event loop, causing anyio (used by httpcore/httpx) to crash with TypeError: cannot create weak reference to 'NoneType' object on every HTTP request
• This broke 58 of 125 tests on the Python 3.14 CI matrix added in 93bc2ca
• Replaced nest_asyncio.apply() + loop.run_until_complete() with a _run_async() helper that uses asyncio.run() directly when no event loop is running, or dispatches to a worker thread when called from within a running loop (preserving Jupyter notebook compatibility)
• Same fix applied to PaginatedResults._get_next_page() which had the same pattern
• Removed nest_asyncio from requirements.txt and setup.py

Test plan

• All 123 tests pass on Python 3.14
• All 123 tests pass on Python 3.12 (backward compatibility)
• Jupyter use case preserved (thread-based fallback when event loop is already running)

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