Switching models before sending the first real message titled the session
"[System: The active model for this chat has…" instead of the user's actual
question.
`_append_model_switch_marker` persists its notice with `role="user"` because
strict OpenAI-compatible providers reject a system message that is not first
(#48338). Titling had no way to tell that apart from a genuine opening turn,
which caused two distinct failures:
1. `_MACHINE_PREFIXES` did not cover the marker. Its `[System: ` prefix
matches none of `[CONTEXT COMPACTION`, `[Runtime note:`, or `[SYSTEM]`
(different case, no closing bracket), so `is_titleable_user_message()`
returned True and the marker was formatted into the title.
2. `maybe_auto_title()` counted the marker as a user message. With the marker
present, the first real question arrived at `user_msg_count == 2` and the
`> 1` guard returned early, so the session was never titled at all and its
`title` column stayed NULL. Fixing only (1) would therefore have traded a
wrong title for a permanently missing one.
Add the marker prefix to `_MACHINE_PREFIXES` (kept in sync with
`tui_gateway.server._MODEL_SWITCH_MARKER_PREFIX`) and count only titleable
user messages when detecting the opening turn.
The guard stays narrow: ordinary user text that happens to start with
"[System:" still titles normally.
Adds 6 regression tests, verified to fail without the fix.
b684cbb094