Remove OpenAI Agents integration

[brianstrauch]

Summary

• remove the bundled OpenAI Agents implementation and its SDK-owned test suite
• retain temporalio[openai-agents] as a forwarding extra that installs temporalio-openai-agents
• preserve the old public temporalio.contrib.openai_agents imports with explicit compatibility modules
• make temporalio.openai_agents the canonical standalone import path
• remove the temporalio.contrib.__getattr__ migration hook
• extend the temporalio package path so editable standalone plugins can be discovered
• remove the dedicated CI step and CODEOWNERS entries for the formerly bundled implementation

This is the SDK half of the OpenAI Agents plugin cutover. The standalone distribution installs under temporalio.openai_agents, so it can coexist with an SDK-bundled temporalio.contrib.openai_agents implementation without sharing files.

Release order and draft status

This PR is intentionally a draft until temporalio-openai-agents is published to PyPI. Until then, the new forwarding dependency cannot be added to uv.lock, so full SDK validation is expected to fail during dependency resolution.

1. Merge Prepare OpenAI Agents package cutover ai-integrations#23 and publish temporalio-openai-agents through its normal release workflow.
2. Regenerate the lockfile and run the full SDK validation suite.
3. Merge this PR and publish the Temporal SDK release.

Migration

Remove the openai-agents extra from the existing Temporal dependency and install the standalone package directly:

# Before
dependencies = [
    "temporalio[openai-agents,otel,pydantic]",
]

# After
dependencies = [
    "temporalio[otel,pydantic]",
]

Remove the brackets if no extras remain. Then install the standalone package:

uv add temporalio-openai-agents

Change application imports to the standalone package:

from temporalio.openai_agents import OpenAIAgentsPlugin

The new import path selects the standalone implementation instead of the SDK-bundled temporalio.contrib.openai_agents implementation.

The forwarding extra and old public imports remain available for compatibility, but direct dependency on temporalio-openai-agents and the temporalio.openai_agents import are the recommended migration.

Validation

Completed against a local build of the standalone package:

• Ruff checks for the changed SDK files
• compatibility re-exports resolve to the standalone public objects

Pending publication of temporalio-openai-agents:

• regenerate and commit uv.lock
• full Poe lint and test suite