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-pi-ai/src/stream.ts

@@ -8,7 +8,7 @@
  * @module dsh-llm-pi-ai/stream
  */
 
-import { CallId, CONTEXT_WINDOW_EXCEEDED_CODE, isContextWindowExceededError, isQuotaExceededError, LlmError, QUOTA_EXCEEDED_CODE } from '@deepseek-ai/dsh-llm'
+import { CallId, CONTEXT_WINDOW_EXCEEDED_CODE, EMPTY_RESPONSE_CODE, isContextWindowExceededError, isQuotaExceededError, LlmError, QUOTA_EXCEEDED_CODE } from '@deepseek-ai/dsh-llm'
 import type { FinishReason, StreamChunk, TokenUsage } from '@deepseek-ai/dsh-llm'
 import { isContextOverflow } from '@earendil-works/pi-ai'
 import type { AssistantMessage, AssistantMessageEvent, Usage as PiUsage } from '@earendil-works/pi-ai'
@@ -48,7 +48,8 @@ function classifyPiAiError(message: string): string {
  * @param contextWindow - resolved catalog capacity for usage-based overflow detection.
  * @returns the mapped harness reason. Recognized error text, `stop` usage above
  *   `contextWindow`, and zero-output `length` usage that fills the window map
- *   to `CONTEXT_WINDOW_EXCEEDED`.
+ *   to `CONTEXT_WINDOW_EXCEEDED`; a `stop` with no content blocks maps to an
+ *   `EMPTY_RESPONSE` error.
  */
 export function mapStopReason(message: AssistantMessage, contextWindow?: number): FinishReason {
   const piAiOverflow = isContextOverflow(message, contextWindow)
@@ -66,7 +67,19 @@ export function mapStopReason(message: AssistantMessage, contextWindow?: number)
   }
 
   switch (message.stopReason) {
-    case 'stop': return { kind: 'stop' }
+    case 'stop':
+      // A terminal stop that produced no content blocks is a degenerate
+      // provider completion, not a successful (empty) assistant message.
+      if (message.content.length === 0) {
+        return {
+          kind: 'error',
+          failure: {
+            message: `model "${message.model}" returned a completed response with no content`,
+            code: EMPTY_RESPONSE_CODE,
+          },
+        }
+      }
+      return { kind: 'stop' }
     case 'length': return { kind: 'max-tokens' }
     case 'toolUse': return { kind: 'tool-calls' }
     case 'aborted': return {