chore: extract remote compaction request attempts

[celia-oai]

Why

This PR is a behavior-preserving refactor only. It does not add a fallback, change which model is used for compaction, or otherwise change compaction behavior. The behavioral change is implemented in the stacked follow-up, #30319.

Pre-sampling compaction deliberately uses the previous turn's context when the compaction compatibility hash changes or when switching to a model with a smaller context window. That preserves the model settings that produced the history being compacted, but the previous context is not always usable. For example, a resumed thread can still reference a model slug that has since been retired, causing compaction to fail before the currently selected model can sample.

#30319 addresses that failure mode by retrying compaction with the current turn's selected model when the backend rejects the previous-model attempt. This PR performs only that preparatory refactor.

What changed

• Extracted one legacy /responses/compact request attempt into compact_remote_request.rs.
• Extracted one Responses-based remote compaction request attempt into compact_remote_v2_attempt.rs.
• Kept hooks, lifecycle events, analytics, window advancement, history processing and installation, and error behavior unchanged in the existing orchestration paths.
• Preserved standalone Responses-based compaction's owned client-session lifetime through lifecycle completion.

Testing

• just test -p codex-core -E 'test(remote_compact)' (22 tests)

[celia-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

Reviewed commit: 1f3200acfa

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