* feat: auto-detect model changes when LLM backend swaps models
The BackendHealthMonitor already probes /v1/models every 30s but
discarded the response. Now compares the detected model against the
last known one and triggers a registry reload when it changes.
- Extract _extract_context_window() helper for reuse across
detect_model, probe_model_endpoint, and the health monitor
- BackendHealthMonitor: new provider/initial_model/on_model_changed
params; _check_model_change() fires callback on model swap
- Server: wire _handle_model_change callback that updates cli_model_args
and calls registry.reload(); guarded by _user_specified_model flag
so --model overrides are never auto-replaced
- Session: _refresh_model_from_registry() called at top of send();
two string compares when nothing changed, full re-resolve on swap
- 7 new tests for _extract_context_window and model change detection
* fix: address Copilot review on model re-detection
- server: update cli_model_args only after successful reload (not
before), add finally block for new_reg.shutdown(), guard against
cli_model_args not yet initialized
- session: wrap registry lookup in try/except for concurrent reload
race, reset judge on model change, recompute tool_truncation when
context_window changes in auto mode
Patrick Buckley
23fed785c4