fix: preserve empty reasoning_content for DeepSeek v4 thinking mode (#399)

DeepSeek v4 in thinking mode sometimes returns reasoning_content: ""
when the model answers directly without internal reasoning. Two places
were filtering the empty string out, which dropped the thinking block
from the assistant turn entirely. The next request then omitted
reasoning_content for that prior turn, and DeepSeek rejected with
400 "reasoning_content ... must be passed back to the API".

Fix:
- openaiStreamAdapter: open a thinking block whenever reasoning_content
  is present (including ""); skip the empty thinking_delta event since
  the empty value is already conveyed by the block's initial state.
- openaiConvertMessages: preserve empty thinking blocks as
  reasoning_content: "" when serializing assistant messages back to
  the OpenAI/DeepSeek format.

Tests:
- New: empty reasoning_content opens a thinking block (adapter).
- Updated: empty thinking blocks now round-trip as reasoning_content: ""
  instead of being dropped.
- New: assistant messages with no thinking block still omit
  reasoning_content (regression guard for non-thinking models).

packages/@ant/model-provider/src/shared/openaiStreamAdapter.ts

@@ -106,9 +106,13 @@ export async function* adaptOpenAIStreamToAnthropic(
     // Skip chunks that carry only usage data (no delta content)
     if (!delta) continue
 
-    // Handle reasoning_content → Anthropic thinking block
+    // Handle reasoning_content → Anthropic thinking block.
+    // Empty string is a valid signal: DeepSeek v4 thinking mode sometimes
+    // returns reasoning_content: "" when the model answers directly. The
+    // empty thinking block must round-trip back to the API in subsequent
+    // requests, otherwise DeepSeek rejects with 400.
     const reasoningContent = (delta as any).reasoning_content
-    if (reasoningContent != null && reasoningContent !== '') {
+    if (reasoningContent != null) {
       if (!thinkingBlockOpen) {
         currentContentIndex++
         thinkingBlockOpen = true
@@ -125,14 +129,16 @@ export async function* adaptOpenAIStreamToAnthropic(
         } as BetaRawMessageStreamEvent
       }
 
-      yield {
-        type: 'content_block_delta',
-        index: currentContentIndex,
-        delta: {
-          type: 'thinking_delta',
-          thinking: reasoningContent,
-        },
-      } as BetaRawMessageStreamEvent
+      if (reasoningContent !== '') {
+        yield {
+          type: 'content_block_delta',
+          index: currentContentIndex,
+          delta: {
+            type: 'thinking_delta',
+            thinking: reasoningContent,
+          },
+        } as BetaRawMessageStreamEvent
+      }
     }
 
     // Handle text content