[codex] Stabilize compact resume and fork snapshot flaky tests

[charley-oai]

Summary

• stabilize compact_resume_after_second_compaction_preserves_history by asserting against the real /responses request order and accepting the preserved post-compaction block even when earlier history precedes it
• stabilize fork_startup_context_then_first_turn_diff_snapshot by waiting for the parent rollout to persist the pre-fork user turn before forking from the JSONL path
• keep both changes scoped to flaky tests

Root cause

• compact_resume_after_second_compaction_preserves_history combined overlapping mount_sse_once_match recorders and indexed into a synthetic request list, so matcher evaluation order could duplicate requests and skew the expected history slice
• fork_startup_context_then_first_turn_diff_snapshot read the fork source rollout immediately after TurnComplete, but the rollout writer persists asynchronously, so the fork sometimes missed the source fork seed turn on disk

Impact

• removes two Linux-sensitive flakes without changing runtime behavior
• keeps the fixes limited to test setup and assertions

Validation

• just fmt
• just argument-comment-lint
• cargo test -p codex-core compact_resume_after_second_compaction_preserves_history -- --nocapture
• repeated cargo test -p codex-core compact_resume_after_second_compaction_preserves_history -- --nocapture 10x
• cargo test -p codex-core fork_startup_context_then_first_turn_diff_snapshot -- --nocapture
• cargo test -p codex-core (the snapshot fix passed; one unrelated failure remained in tools::js_repl::tests::js_repl_imported_local_files_can_access_repl_globals because mktemp was denied during dotslash setup)

[charley-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".