[26.0] Fix event loop blocking in async API routes

[mvdbeek]

Async def route handlers were calling synchronous DB queries, DNS lookups, and HTTP APIs directly on the event loop, blocking all concurrent requests on the same worker.

• chat.py: use await agent.run() instead of agent.run_sync(), use AsyncOpenAI instead of sync openai client, wrap all chat_manager and job_manager DB calls with anyio.to_thread.run_sync
• agents.py: wrap chat_manager/job_manager DB calls in analyze_error and create_custom_tool with anyio.to_thread.run_sync
• proxy.py: wrap _validate_url_and_access (blocking socket.getaddrinfo) with anyio.to_thread.run_sync
• error_analysis.py: wrap job_manager.get_accessible_job with anyio.to_thread.run_sync

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.