## 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 Chen
1f710c973b