fix(server): fail V2 turns when the OpenCode event stream ends

[saphid]

An OpenCode event stream ending cleanly could leave its active turn running forever. Treat unexpected EOF as a transport failure, finish active turns, retire the runtime, and reject turns racing shutdown.

Rebuilt on current V2 7dd9873ed with the test-only prerequisite #10865 (merge that first). Current head 1b789282c; the feature remains two files.

Validation: 59 focused adapter and persistence tests pass, plus server typecheck, targeted lint and formatting. A follow-up regression reproduces a compaction response racing EOF: it fails on the previous code, while all 25 adapter tests now pass with termination owning the failure classification. Server typecheck and targeted lint/formatting pass again. New CI is pending. Cross-provider review was unavailable: the direct Claude Opus 5 high launcher exited 1 with expired OAuth before inference; no Claude review occurred.

Targets V2 #2829. Updated with GPT-6 in Codex/T3.

[cursor]

Stale Bugbot comment from a previous run.

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Approvability

Verdict: Approved at 1b78928

Macroscope's review found this PR approvable — This is a narrowly scoped OpenCode V2 error-handling fix that cleanly terminates active turns and blocks stale work after event-stream EOF. The production change is isolated to the adapter and is covered by deterministic tests for clean termination and race conditions.

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Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

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Dismissing prior approval to re-evaluate 02f7c19

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