refactor(gateway): extract message/config helpers from run.py (slice 2 of #54962)

[andrexibiza]

Related #54962 #55138 #77433

What does this PR do?

Second slice of the Extract Gateway Platform Routing from
gateway/run.py refactor (#54962, #55138) — follow-up to #77433 (slice 1:
display helpers).

This slice pulls the message/config helper cluster out of the
26,700-line god-file into gateway/message_config_helpers.py:

• _csv_or_list_to_set — config list / comma-scalar normalization
• _slack_ignored_channels_from_gateway_config — runner-level Slack
  channel blacklist guard (fail-safe below the adapter's first-line drop)
• _slack_parent_channel_id — thread-scoped chat ID → parent channel
• _is_slack_ignored_channel — the dispatch-time blacklist check
• _message_timestamps_enabled — gateway.message_timestamps opt-in

No behavior change: pure move + import. gateway/run.py shrinks by
65 lines; the extracted functions are module-attribute-imported so
existing tests that reference them via gateway.run stay green.

The extracted module has no module state — everything derives from
arguments, so the helpers are directly unit-testable (the existing
test_message_timestamps.py and test_slack_runner_ignored_channels.py
cover them; 5 pure-logic tests pass on this branch).

Scope honesty

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

How to test

pytest tests/gateway/test_message_timestamps.py tests/gateway/test_slack_runner_ignored_channels.py -q
# 5 passed; 1 env-dependent test requires pytest-asyncio (not in this venv,
# runs in CI where the plugin is installed)

What platforms were tested?

• Windows 11 native: both files parse, imports resolve, 5 pure-logic
  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 from 26,819 to 26,600
lines across slices 1-2, 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 (5 pass)
• git diff --check clean
• Attribution audit clean

Part of #78647

[andrexibiza]

Update: this PR now also carries slice 3 (same branch, second commit).

Slice 3 — resume/replay helpers (gateway/resume_replay_helpers.py)

• build_resume_recovery_note — 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 — assistant-message
  replay field whitelist (reasoning/thinking round-trip fidelity)
• gateway/run.py: -172 lines this slice (total -237 across slices 2-3)
• Byte-identical extraction — AST-verified against origin/main, zero
  behavior change
• 43 tests pass (replay-entry + resume-pending + message-timestamps suites)

Slices landed on this branch: 2 (message/config, #77438) + 3 (resume/replay).
Slice 1 (display helpers) is PR #77433. Continuing toward #54962.