Generalize shared multi-user session handling so non-thread group sessions
(group_sessions_per_user=False) get the same treatment as shared threads:
inbound messages are prefixed with [sender name], and the session prompt
shows a multi-user note instead of pinning a single **User:** line into
the cached system prompt.
Before: build_session_key already treated these as shared sessions, but
_prepare_inbound_message_text and build_session_context_prompt only
recognized shared threads — creating cross-user attribution drift and
prompt-cache contamination in shared groups.
- Add is_shared_multi_user_session() helper alongside build_session_key()
so both the session key and the multi-user branches are driven by the
same rules (DMs never shared, threads shared unless
thread_sessions_per_user, groups shared unless group_sessions_per_user).
- Add shared_multi_user_session field to SessionContext, populated by
build_session_context() from config.
- Use context.shared_multi_user_session in the prompt builder (label is
'Multi-user thread' when a thread is present, 'Multi-user session'
otherwise).
- Use the helper in _prepare_inbound_message_text so non-thread shared
groups also get [sender] prefixes.
Default behavior unchanged: DMs stay single-user, groups with
group_sessions_per_user=True still show the user normally, shared threads
keep their existing multi-user behavior.
Tests (65 passed):
- tests/gateway/test_session.py: new shared non-thread group prompt case.
- tests/gateway/test_shared_group_sender_prefix.py: inbound preprocessing
for shared non-thread groups and default groups.
