[Bug]: Incomplete assistant reply is persisted as `complete` when the model output stream ends prematurely

Summary

An assistant reply stopped mid-generation (the text ends at the title of list item 4, with the markdown list left unclosed), but the session persistence layer recorded the turn as finishReason=complete / status=completed / hasFinalResponse=true. The UI shows no "reply may be incomplete" indication, so the user cannot distinguish an intentional stop from a transport/generation failure. This is a silent failure with the same observability gap as session freeze / review freeze issues.

Expected behavior: when the model output stream ends prematurely (provider-side interruption, timeout, connection drop, or max_tokens cut), the session pipeline should record a non-complete finish reason (length / interrupted / error), persist the turn as incomplete, and let the UI surface a hint (e.g. "reply may be incomplete") instead of silently marking it complete.

Area

Agent runtime / core (session persistence and finishReason determination); AI provider / model adapter (premature stream termination).

Reproduction or evidence

Hard to reproduce deterministically (it depends on the underlying model stream ending early), but this is a real occurrence with complete local evidence from the persisted turn data (no private data included):

1. Phenomenon: the assistant reply was cut mid-sentence — the text stops at 4. **压缩/总结(co, right after the "能结构性解决" table and list items 1-3 of "解决不了的(方案边界)". Item 4 has a title only, no body; the markdown list is unclosed.

2. Persisted evidence (local sessions/<session-id>/turns/turn-0004.json):

   • textItems[0].content is only 865 characters, ending at 4. **压缩/总结(co;
   • the same modelRound's thinkingItems are complete, containing the full analysis for items 8, 9, 10 and the conclusion — proving the model finished reasoning and had a full answer ready, but the text stream terminated mid-generation;
   • the turn records finishReason=complete, status=completed, hasFinalResponse=true — no truncation or error marker at all;
   • tokenUsage.outputTokens=1776 (including thinking); the text is only a fraction of that, so the output stream stopped far earlier than a normal completion point.

3. Control group: other turns in the same session with much longer replies (e.g. 4908 and 2787 characters) completed normally. This shows it is not "long replies always get cut"; it is an intermittent premature stream termination.

Suggested fix

1. When the response stream ends early (missing stream-end frame, finish_reason = length/interrupted, or visibly unclosed text), mark the round as interrupted/incomplete instead of complete.
2. Keep finishReason semantics distinct: complete / length / interrupted / error, and retain the termination reason in the round state or tokenUsage.
3. Surface a "reply may be incomplete" hint in the UI and allow the user to regenerate or continue.
4. Optionally add a persistence-layer integrity check (e.g. warn when thinking is complete but the text is clearly truncated).

Environment, if relevant

• BitFun v0.2.15 (2026-07-31 release), Windows desktop app, built-in personal-assistant session.
• Not tied to a specific model/provider; any provider stream that ends early during a long output can trigger this.