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/openaiConvertMessages.ts

@@ -206,12 +206,14 @@ function convertInternalAssistantMessage(
         },
       })
     } else if (block.type === 'thinking') {
-      // DeepSeek thinking mode: always preserve reasoning_content.
-      // DeepSeek requires reasoning_content to be passed back in subsequent requests,
-      // especially when tool calls are involved (returns 400 if missing).
+      // DeepSeek thinking mode: always preserve reasoning_content,
+      // including the empty-string case. DeepSeek v4 may return
+      // reasoning_content: "" when the model answers directly, and the
+      // empty value must be echoed back in the next request — otherwise
+      // DeepSeek returns 400 ("reasoning_content ... must be passed back").
       const thinkingText = (block as unknown as Record<string, unknown>)
         .thinking
-      if (typeof thinkingText === 'string' && thinkingText) {
+      if (typeof thinkingText === 'string') {
         reasoningParts.push(thinkingText)
       }
     }