fix(gateway): always prefix non-DM messages with sender name

[Wintle]

Summary

The LLM never saw sender attribution on group / thread messages under the
default group_sessions_per_user=True config. The prefix code in
GatewayRunner._prepare_inbound_message_text only fired when
is_shared_multi_user_session(source) was True, and that predicate is
False by default — session keys isolate per-participant, so the LLM saw
plain text without any [user_name] prefix.

Decouple the two concerns

is_shared_multi_user_session describes session-key isolation (whether
multiple participants share the same conversation history). The sender
prefix describes message attribution (whether the LLM can tell who
sent the current message). Those are independent questions and conflating
them was the bug.

New predicate _should_prefix_sender:

source.chat_type != "dm" and bool(source.user_name)

Prefix every non-DM inbound (group + thread), regardless of
group_sessions_per_user / thread_sessions_per_user config. DMs keep
the existing 1:1 contract — no prefix.

Backward compatibility

• _is_shared_multi_user is still computed at the top of the block (kept
  as a named local so any future consumer in the same scope that expects
  it does not break). It is no longer used for the prefix decision.
• DM messages: identical behaviour to before (no prefix).
• Group/thread with non-shared sessions: NEW — prefix now fires.
  The previous behaviour was a latent bug; if a downstream caller depended
  on the absence of a prefix on isolated group sessions it was depending
  on a bug.
• source.user_name None / empty: no prefix (same as before).
• Hostile display names containing newlines or control chars: still
  neutralized via neutralize_untrusted_inline_text before interpolation
  (same defence that already existed for shared sessions).

Tests

tests/gateway/test_sender_prefix_decoupling.py (new, 6 tests):

• group message under default isolation gets prefix (the bug)
• thread message gets prefix
• DM message never gets prefix
• user_name=None does not produce a prefix
• hostile display names with newlines are neutralized
• Slack channel adds the verifiable <@U...> user id next to the display
  name (existing behaviour preserved)

Verification

RED on origin/main (stashed source fix, kept tests):

$ git stash push -- gateway/run.py
$ ./venv/bin/python -m pytest tests/gateway/test_sender_prefix_decoupling.py
5 failed, 1 passed

GREEN after fix (popped stash):

$ git stash pop
$ ./venv/bin/python -m pytest tests/gateway/test_sender_prefix_decoupling.py
6 passed

Sibling regression (test_internal_event_never_interrupts_busy_session.py
and other gateway tests that exercise _prepare_inbound_message_text):
all green.

Files changed

• gateway/run.py — new module-level helper _should_prefix_sender(source);
  _prepare_inbound_message_text uses it instead of the is_shared_multi_user
  gate. _is_shared_multi_user local kept for backward compat (still
  computed, no longer consulted for prefixing).
• tests/gateway/test_sender_prefix_decoupling.py (new) — 6 invariant tests
  pinning the prefix-on-group / no-prefix-on-DM / neutralization contract.

Bot / duplicate check

• Open issues referencing this: none found
• Open PRs referencing this: none found (related-but-distinct: fix(matrix): propagate sender MXID + reply context to MessageEvent #80206
  addresses the same root-cause surface area from the message-event side;
  not a duplicate, both stand alone)