Fix pre-connect Realtime manager event transforms

[fengjikui]

Fixes #3402.

Summary

• route queued Realtime manager events through the same transform path used by connected sync sends
• strip Omit values from dict events before serializing them into the pre-connect send queue
• add sync and async manager regression coverage for response.cancel with event_id=omit

Validation

• env HTTP_PROXY= HTTPS_PROXY= ALL_PROXY= NO_PROXY='*' http_proxy= https_proxy= all_proxy= no_proxy='*' uv run --with pytest --with pytest-asyncio --with pytest-xdist --with dirty-equals --with respx --with time-machine --with 'httpx[socks]' --with aiohttp --with httpx-aiohttp pytest tests/api_resources/test_realtime.py -q
• uv run --with ruff ruff format --check src/openai/resources/realtime/realtime.py tests/api_resources/test_realtime.py
• uv run --with ruff ruff check src/openai/resources/realtime/realtime.py tests/api_resources/test_realtime.py
• git diff --check