fix(llm): classify empty model completions as retryable EMPTY_RESPONSE

A well-formed provider stream that ends with finish_reason stop and zero
content blocks previously became a successful empty assistant message: the
turn completed silently, and drivers like goal-session counted the no-op
round. Both adapters now map that degenerate completion to a finish
{kind:'error'} with the new canonical EMPTY_RESPONSE code from dsh-llm, and
dsh-llm-retry adds the code to its default retryable set, so the existing
closed-step recovery path retries it and fails loud once the budget is
exhausted.

Covered by adapter unit tests, an llm-retry default-policy test, and a new
authored keyless ACP snapshot (empty-response-retry) with a deterministic
1 ms zero-jitter retry overlay.

packages/llm/llm-deepseek/src/translate.ts

@@ -8,7 +8,7 @@
  * @module dsh-llm-deepseek/translate
  */
 
-import { CallId, LlmError } from '@deepseek-ai/dsh-llm'
+import { CallId, EMPTY_RESPONSE_CODE, LlmError } from '@deepseek-ai/dsh-llm'
 import type { ContentBlock, FinishReason, StreamChunk, TokenUsage } from '@deepseek-ai/dsh-llm'
 import { DONE } from './sse.ts'
 import type { WireChunk, WireUsage } from './types.ts'
@@ -80,6 +80,8 @@ function closeBlock(block: OpenBlock): ContentBlock {
  * Malformed JSON payloads abort the stream with `MALFORMED_RESPONSE`.
  * @param payloads - SSE data payloads from {@link parseSse}, `[DONE]`-terminated.
  * @returns deltas as they arrive; `block-end`s, `usage`, and `finish` are all deferred to the `[DONE]` sentinel.
+ *   A `stop` (or absent) finish with no opened blocks is a degenerate provider completion and maps to an
+ *   `EMPTY_RESPONSE` error finish instead of a successful empty message.
  */
 export async function* translate(payloads: AsyncIterable<string>): AsyncGenerator<StreamChunk> {
   let nextIndex = 0
@@ -102,7 +104,16 @@ export async function* translate(payloads: AsyncIterable<string>): AsyncGenerato
         yield { type: 'block-end', index: block.index, block: closeBlock(block) }
       }
       if (pendingUsage) yield { type: 'usage', usage: pendingUsage }
-      yield { type: 'finish', reason: pendingFinish ?? { kind: 'stop' } }
+      const reason = pendingFinish ?? { kind: 'stop' as const }
+      yield {
+        type: 'finish',
+        reason: reason.kind === 'stop' && order.length === 0
+          ? {
+            kind: 'error',
+            failure: { message: 'model returned a completed response with no content', code: EMPTY_RESPONSE_CODE },
+          }
+          : reason,
+      }
       return
     }