fix(cli): preserve empty reasoning_content in interleaved transform

DeepSeek thinking mode may return empty reasoning_content on assistant
messages in a tool call chain, which must be sent back in subsequent
requests. The interleaved transform's truthy check was dropping it.

Always set providerOptions.openaiCompatible[field] for assistant
messages so the AI SDK's metadata spread carries it through.

Cherry-picked from anomalyco/opencode#24146 (931e6ed).

Author: heimoshuiyu

packages/opencode/src/provider/transform.ts

@@ -193,24 +193,19 @@ function normalizeMessages(
         // Filter out reasoning parts from content
         const filteredContent = msg.content.filter((part: any) => part.type !== "reasoning")
 
-        // Include reasoning_content | reasoning_details directly on the message for all assistant messages
-        if (reasoningText) {
-          return {
-            ...msg,
-            content: filteredContent,
-            providerOptions: {
-              ...msg.providerOptions,
-              openaiCompatible: {
-                ...msg.providerOptions?.openaiCompatible,
-                [field]: reasoningText,
-              },
-            },
-          }
-        }
-
+        // Include reasoning_content | reasoning_details directly on the message for all assistant messages.
+        // Always set the field even when empty — some providers (e.g. DeepSeek) may return empty
+        // reasoning_content which still needs to be sent back in subsequent requests.
         return {
           ...msg,
           content: filteredContent,
+          providerOptions: {
+            ...msg.providerOptions,
+            openaiCompatible: {
+              ...msg.providerOptions?.openaiCompatible,
+              [field]: reasoningText,
+            },
+          },
         }
       }