fix: surface when a response is truncated by the output limit

[northwang-lucky]

The problem (observed in real use). When an assistant turn ends on stopReason: "length" — e.g. a thinking model spending the entire output budget on reasoning — the conversation stops with no visible answer and no warning; the UI just sits on the last thinking block. It reads exactly like a hung session. We hit this with a 128K-token reasoning-only truncation on a long session: the model never produced a final message, and nothing in the UI said so.

The fix. Mirrors the existing provider-error handling:

• getAssistantTruncationNotice() in lib/message-display.ts — a pure function beside getAssistantErrorMessage(): non-streaming + stopReason === "length" -> a short notice. stopReason reaches MessageView intact on both the live event path (normalizeToolCalls preserves it through the spread) and the history path (verified).
• Rendered as an inline notice beside the existing error banner (MessageView), including the thinking-only-truncated case; classified alongside errors in the final-answer logic (ChatWindow) so the message is never silently dropped.

Notice copy: "This response was cut off after reaching the model's output limit. Send a follow-up to continue."

Tests. 5 new cases (3 on the pure function covering the discriminating states, 2 real-render cases on MessageView including thinking-only). tsc / eslint clean; 1025 tests pass on this branch.

Found while building the programmatic dispatch API discussed in #829 — unrelated to that design, so it ships standalone.

[agegr]

Real gap: a stopReason: "length" turn with reasoning-only output looked exactly like a hung session. Mirroring the provider-error path is the right shape, and the ChatWindow final-answer classification fix means the message is not silently dropped.

One change pushed to the branch: the notice copy was a hardcoded English string in lib/message-display.ts; all UI copy here goes through i18n. The helper is now isAssistantTruncated(message, opts): boolean and MessageView renders t("chat.truncatedByOutputLimit") (en / zh-CN / zh-TW added). Tests adjusted; the render tests still assert the English copy through I18nProvider.

Merged current main; tsc, eslint, and message-display / MessageView / ChatWindow tests pass. Approving.