feat: #2206 Add responses.compact: auto-compact long conversations

[ruskaruma]

Summary

Implements OpenAIResponsesCompactionSession to add seamless support for the newly added responses.compact API, matching the TypeScript SDK implementation (openai-agents-js#760).

Core Implementation

• OpenAIResponsesCompactionSession: Session decorator that automatically compacts conversation history when it grows too large
• CompactionItem: New item type to handle encrypted compaction content
• OpenAIResponsesCompactionAwareSession: Protocol extension for compaction-aware sessions
• is_openai_responses_compaction_aware_session(): Type guard for runtime checks

Key Features

• Configurable should_trigger_compaction hook for custom compaction logic
• Default threshold of 10 candidate items (excludes user messages and existing compaction items)
• Model validation to ensure only OpenAI Responses API–compatible models are used
• Automatic runner integration, compaction runs after each turn if threshold is met
• Chat completions guardrail to prevent unsupported usage

Usage Example

from agents import Agent, MemorySession, OpenAIResponsesCompactionSession, Runner

session = OpenAIResponsesCompactionSession(
    session_id="my-session",
    underlying_session=MemorySession(),
)

result = await Runner.run(agent, "Hello!", session=session)

Test plan

• Added 17 new tests in tests/memory/test_openai_responses_compaction_session.py

Issue number

Closes #2206

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[ruskaruma]

Greetings @seratch and @ihower, this PR implements the core responses.compact support, aligned with the TypeScript SDK implementation (#760).

Following the TS SDK approach, a few related enhancements could either be handled in follow-up PRs or incorporated here, depending on your preference:

• Usage tracking integration (similar to #785)
• Documentation updates (similar to #784)

I’d appreciate your feedback on whether this aligns with the intended design for the issue, and I’m happy to adjust the implementation as needed.

[seratch]

Thanks for sending this. This looks like a good port of the TS implementation I did. I will review the details early next year.