FIX Harden prompt target cancellation cleanup

[Roman Lutz (romanlutz)]

Description

Cancellation during provider setup or response handling can leave shared work cancelled or background resources running. This change makes target cleanup deterministic without changing the provider-attempt design.

• Shield Hugging Face model loading so cancelling one send does not cancel the shared load task for other callers.
• Remove and close partially configured OpenAI Realtime connections when setup is cancelled or fails.
• Cancel and await unfinished Realtime receive tasks when text or audio sends fail, preventing orphaned listeners.

Tests and Documentation

• Added unit coverage for cancellation during Realtime session configuration.
• Added unit coverage for receive-task cleanup after response creation fails.
• Added unit coverage confirming send cancellation preserves shared Hugging Face model loading.
• Ran both focused target test modules: 120 passed.
• Ran Ruff check and format checks, changed-file pre-commit hooks including ty, and git diff --check.
• Documentation: N/A. This is an internal lifecycle cleanup fix with no public API change.