fix(sse): keep a clean empty Claude end_turn

[HouMinXi]

Summary

• A streamed Claude turn that finishes with message_stop, stop_reason end_turn or stop_sequence, and no content block was turned into 502 Claude returned an empty response (no content block). Claude Code's prompt-suggestion request asks for a short guess or nothing, so an empty answer is valid.
• message_delta.delta.stop_reason is now kept on the stream lifecycle. The flush check leaves that finished empty turn alone. content_filter, a missing stop reason, a lifecycle that never reaches message_stop, and a zero-byte stream still abort.
• Related to fix(claude): a clean Claude end_turn with no content block becomes a 502 (breaks Claude Code prompt suggestions) #15488

Validation

• node --import tsx/esm --test tests/unit/stream-claude-empty-end-turn.test.ts tests/unit/stream-empty-response-retry-96ef4a.test.ts — 12 pass, 0 fail
• Disabling the stopReason write makes only the wiring test fail. The decision tests stay green.
• Four LOCAL forge runs on the uncommitted diff, backend review-default. All four had 0 CONFIRMED. Round 2 was PASS with clean 2. Rounds 1, 3, and 4 were PENDING because of UNCERTAIN notes: a guessed top-level stop_reason (fixtures in this repo use delta.stop_reason), a null payload (the type check already returns), and a claim that max_tokens / tool_use should also pass. Those two stay failures on purpose. One note was an excerpt line count.

Tests Added Or Updated

• tests/unit/stream-claude-empty-end-turn.test.ts
  • end_turn and stop_sequence with a complete lifecycle do not abort
  • content_filter, a missing stop reason, a missing message_stop, and a zero-byte stream still abort
  • message_delta stores delta.stop_reason, and that stored value is what the decision reads

Coverage Notes

• The tests call the decision and the lifecycle update. They do not open a live Claude stream.

Reviewer Notes

• The non-streaming empty-body check is unchanged. fix(sse): widen max_tokens/tool_use exemption to lone empty text blocks (#12968) #13771 keeps an empty end_turn flagged there on purpose.
• fix(providers): Codex streaming request returns 502 "Provider returned empty content" in a long OpenCode session #15489 (Provider returned empty content on a Codex stream) is a different check. Its report did not include the upstream finish reason, so stop was not added to that allowlist.