The default-only context_length guard compared model.default to the session
model with exact string equality. But model.default and the session model can
be stored in equivalent-but-different shapes (bare 'claude-opus-4.8',
provider-prefixed 'anthropic/claude-opus-4.8', or '@anthropic:claude-opus-4.8').
An exact compare wrongly treats the actual default model as non-default and
drops its configured context_length cap for provider-prefixed configs.
Add api/routes._model_matches_configured_default(session_model, cfg_default,
provider) that normalizes all three shapes, and use it at all 6 guard sites
(routes resolver + the 5 api/streaming.py sites: live-usage snapshot, persistence
_skip_cc_cl, persistence fallback _apply_cfg_ctx, SSE-done _dropped_stale_cap_sse,
SSE fallback _apply_cfg_ctx). Imported function-scoped in streaming to avoid the
routes<->streaming module-level circular import. 10 helper unit tests + a
behavioral test that a prefixed default still receives its cap.
Co-authored-by: allenliang2022 <allenliang2022@users.noreply.github.com>
