Improve ChatGPT subscription response resilience

[morgankrey]

Summary

This started from #57636, after we saw ChatGPT subscription/Codex requests stall over the past week. OpenCode v1.15.11 shipped related resilience fixes for the same class of Codex subscription endpoint issues, so this ports the relevant pieces into Zed's native ChatGPT subscription provider.

When Zed asks ChatGPT/Codex for a response, sometimes the server connection can get stuck before it even sends the first response headers. Before this PR, Zed could wait indefinitely, which looks like OpenCode/Zed “stalling.”

This PR makes Zed:

• Wait up to 10 seconds for the server to start responding.
• If nothing comes back in that window, treat it as a temporary network/API failure.
• Let the existing retry logic try again instead of leaving the user stuck.
• Send a stable session-id header so OpenAI’s Codex backend can associate requests with the same Zed agent thread.
• Add tests to make sure:
  • stuck-before-response requests time out,
  • normal slow streaming responses are not cut off,
  • ChatGPT subscription requests send the right session header,
  • the agent retries this kind of failure.

intended user-facing result is: fewer “the assistant is just sitting there forever” failures when using ChatGPT subscription models.

Verification

• cargo test -p open_ai responses
• cargo test -p language_models openai_subscribed
• cargo test -p agent test_send_retry_on_http_send_error
• cargo check -p open_ai
• cargo check -p language_models
• cargo check -p agent

Release Notes:

• Fixed ChatGPT subscription requests stalling indefinitely before response headers arrive.

[osyvokon]

I'm okay with this as a workaround, but I think we should revisit it in a few weeks and rollback when OpenAI fixes their issue

[alterdaemon]

I can't ever enable ChatGPT subscription for the pasts several versions of Zed including the latest one, always getting "Failed to save credentials. Please try again" ... Codex CLI works fine, but auth via subscription don't.