refactor(gateway): extract auto-continue helpers from run.py (slice 8 of #54962) - #77706

Related #54962 #55138

What / Why

Slice 8 of the gateway/run.py god-file extraction campaign (#54962): a pure
move of the auto-continue noise / history-media cluster out of the
26,823-line messaging gateway module into a focused new module,
gateway/auto_continue_helpers.py.

Moved verbatim (zero behavior change, no refactor, docstrings/comments kept):

• _last_transcript_timestamp
• _is_auto_continue_noise
• _strip_auto_continue_noise
• _collect_auto_append_media_tags
• _collect_history_media_paths (with nested _add_text_media_paths)
• exclusive constants: _AUTO_CONTINUE_NOTE_PREFIX, _AUTO_CONTINUE_FALLBACK_PREFIX,
  _AUTO_APPEND_MEDIA_TOOL_NAMES, _JSON_MEDIA_TOOL_PATH_FIELDS, _TOOL_MEDIA_RE

gateway/run.py now re-exports the five functions via a top-of-file
module-attribute import, so every existing gateway.run.<name> reference
(tests, gateway/platforms/base.py) stays green without edits.

How to test

# from the repo root, with the project venv active:
python -c "import gateway.run, gateway.auto_continue_helpers"
python -m pytest tests/gateway/test_73771_media_resend_dedup.py \
  tests/gateway/test_media_extraction.py \
  tests/gateway/test_media_spaced_paths_and_history_dedupe.py \
  tests/gateway/test_restart_resume_pending.py \
  tests/gateway/test_run_tool_media_re.py -q --no-header -p no:cacheprovider

Expected: import gateway.run succeeds (no circular import), all five names
resolve as gateway.run.<name> attributes, and the five referencing test
suites pass. On this machine: 65 passed; the 2 failures in those files
(test_streamed_explicit_media_resend_is_delivered,
test_quoted_spaced_home_path_is_collected_in_delivery_form) are
pre-existing Windows path-normalization environment failures — verified by
stash-proving the identical 2 failures against pristine main.

Platforms tested

• Windows 10 (native) — import gateway.run + targeted pytest suites green,
  scripts/check-windows-footguns.py clean on both files, git diff --check clean.

Shrink

• gateway/run.py: 26,823 → 26,597 lines (−234 moved, +8 import lines; net −226)

Part of #54962
Part of #55138

Part of #78207
Part of #78647