fix: contextvars propagation in ThreadPoolExecutor calls

[galkleinman]

When using ThreadPoolExecutor to run LLM calls concurrently with
guardrails, contextvars were not being propagated to the worker thread.
This caused issues with trace context and other context-dependent
functionality being lost across thread boundaries.

The issue broke the traces of: https://github.com/traceloop/openllmetry

Changes:

• Use copy_context() before submitting tasks to ThreadPoolExecutor
• Apply fix to chat.completions.create, responses.create, and responses.parse
• Add tests to verify contextvars propagate correctly across threads
• Add tests for single and multiple contextvars propagation