refactor(gateway): extract resume/replay helpers from run.py (slice 3 of #54962)

[andrexibiza]

Related #2974 #54962 #55138 #57056 #77433 #77438

What does this PR do?

Third slice of the Extract Gateway Platform Routing from
gateway/run.py refactor (#54962, #55138) — standalone PR for the
resume/replay helper cluster (follow-up to #77433 slice 1 and #77438
slice 2).

This slice pulls the resume/recovery + message-replay helpers out of the
god-file into gateway/resume_replay_helpers.py:

• build_resume_recovery_note — the resume-pending recovery system note
  (interactive vs non-interactive guidance, Gateway auto-resume on a non-interactive platform (webhook) acknowledges restoration instead of completing the interrupted turn #57056)
• _build_replay_entry + _ASSISTANT_REPLAY_FIELDS — the
  assistant-message replay field whitelist (reasoning/thinking-mode
  round-trip fidelity, PR feat: persist reasoning across gateway session turns (schema v6) #2974 contract)

No behavior change: byte-identical extraction (AST-verified against
origin/main). gateway/run.py shrinks by 170 lines; the helpers are
imported at the extraction point so the call sites (resume scheduling,
replay building) and existing tests stay green.

Scope honesty

The full platform-routing extraction from a 26.7K-line file is a
multi-PR effort. This is slice 3, standalone, using the same verified
pattern as slices 1-2 (module + import + shrink + AST-fidelity).
Platform adapters and the dispatch loop remain for follow-up slices.

How to test

pytest tests/gateway/test_replay_entry_fields.py tests/gateway/test_restart_resume_pending.py -q
# 40 passed

What platforms were tested?

• Windows 11 native: both files parse, imports resolve, both functions
  AST-identical to origin/main, 40 tests pass, git diff --check clean,
  attribution audit clean.

Why this matters

Every slice shrinks the largest file in the codebase and gives extracted
helpers direct coverage. gateway/run.py is down ~200 lines across
slices 1-3, with more to come.

Part of #54962
Part of #55138 (Extract Gateway Platform Routing)

• Refactor (no behavior change)
• Bug fix
• Breaking change

Checklist

• Code follows repo style (extraction, no new deps)
• Self-review complete
• Existing tests cover the extracted helpers (40 pass)
• git diff --check clean
• Attribution audit clean

Part of #78647

[andrexibiza]

FILE-LIST coordination — _has_replay_payload carry-forward.

#86050 (fix(gateway): preserve reasoning-only assistant turns on reload) adds _has_replay_payload in gateway/run.py immediately beside _build_replay_entry; it derives its predicate by iterating _ASSISTANT_REPLAY_FIELDS. This slice moves exactly that contract (_ASSISTANT_REPLAY_FIELDS, _build_replay_entry) into gateway/resume_replay_helpers.py.

Merge-order handling: if #86050 lands first, this slice's extraction set must include _has_replay_payload so the helper isn't stranded in the godfile; if this slice lands first, #86050's helper resolves against the re-imported names unchanged. Either order is compatible — this note pins the carry-forward so the extraction stays complete. No line-level overlap with #86050's dispatch arm; git context matching resolves adjacency.