fix(streaming): detect truncated streams and complete promptly on [DONE]

[Fahad090NP]

Summary

The shared streaming engine (src/transports/engine.ts) only ended a response stream on connection close or user cancellation, ignoring OpenCode's data: [DONE] terminator (confirmed in tmp/opencode-dev/.../server/transport/ws.ts, which enqueues data: [DONE]\n\n then closes). This produced two silent failure modes that looked like "the model stopped working":

1. Hang-then-cancel — if the gateway kept the connection alive after [DONE], the read loop never saw done and sat until the 2-minute idle timeout (user cancelled manually, no error shown).
2. Silent truncation — if the connection dropped before [DONE] (proxy reset, upstream crash, truncated payload, VPN/firewall cut), reader.read() returned done with no [DONE] and no finish_reason; the engine treated it as a successful empty response.

Fix

• src/transports/sse.ts — parseServerSentEvent now takes an onDone callback fired on data: [DONE] (and [DONE] is no longer passed to the extractor).
• src/transports/engine.ts — tracks streamFlags.sawDone and breaks the read loop as soon as [DONE] arrives (prompt completion instead of waiting on a lingering socket); after the loop, isStreamTruncated throws a clear OpenCodeRequestError when the stream ended with neither [DONE] nor a captured finish_reason after content was already received, instead of silently succeeding.
• src/transports/sse.ts — isStreamTruncated is a small pure helper (exported, unit-tested).
• src/test/sse.test.ts — unit tests for [DONE] handling and isStreamTruncated.

Verification

• npm run lint (editorconfig, eslint, markdown, prettier, shellcheck, typecheck, unit tests) — green.
• src/test/sse.test.ts — new tests pass.

Builds on the resilience work in #169 (history-trim + transient fetch-retry).