openai-agents-python v0.18.1 adds GPT-5.6 defaults and fixes logprob accumulation

The openai-agents-python library released version 0.18.1, introducing support for the new GPT-5.6 model and addressing several bugs related to session management and performance.

• Adds GPT-5.6 model defaults and migrates examples.
• Fixes cache-write usage across different OpenAI Python versions.
• Makes Realtime session cleanup deterministic.
• Preserves nested tool state during restoration.
• Closes Chat Completions streams on early exit.
• Handles list content in AdvancedSQLiteSession browsing helpers.
• Accumulates streamed logprobs in place to avoid O(n^2) copies.
• Sends logprobs=True when top_logprobs is set on Chat Completions.

This release ensures compatibility with the latest OpenAI models and improves stability for users relying on streaming and session persistence features.