Add timeout for remote compaction requests

[jif-oai]

Why

Remote compaction currently sends a unary POST /responses/compact and waits for the full response before replacing history or emitting the completed ContextCompaction item. Unlike normal /responses streaming requests, this unary compact request had no timeout boundary. If the backend accepts the request and then stalls before returning a body, the existing request retry policy never sees a transport error, so the compact turn can remain stuck after the started item with no completion or actionable error.

That matches the reported hang shape in issues such as #18363, where logs show responses/compact was posted but no corresponding compact completion followed. A bounded request timeout gives the existing retry policy a concrete timeout error to retry instead of letting the user sit indefinitely on automatic context compaction.

What

• Add a request timeout to legacy /responses/compact calls.
• Size that timeout from the provider stream idle timeout with a conservative multiplier, so the default compact attempt gets 20 minutes rather than the 5 minute stream idle window.
• Map API transport timeouts to a request timeout error instead of the child-process timeout message.

Testing

• Not run (per request; CI will cover).

[jif-oai]

@codex review

[chatgpt-codex-connector]

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[rhan-oai]

thanks!