fix(provider): set providerOptions for string-content assistant messages in reasoning fallback

The fallback transform only set reasoning_content in providerOptions for
array-content messages. String-content assistant messages (e.g., "It's 4.")
were converted to array form but didn't get providerOptions set.

Now both content types get reasoning_content: "" injected with the correct
SDK key, ensuring DeepSeek's API receives it on all assistant turns.

Author: knefenk

packages/opencode/src/provider/transform.ts

@@ -246,9 +246,17 @@ function normalizeMessages(
         }
       }
       if (typeof msg.content === "string") {
+        const sdk = sdkKey(model.api.npm) ?? "openaiCompatible"
         return {
           ...msg,
           content: [{ type: "text" as const, text: msg.content }, { type: "reasoning" as const, text: "" }],
+          providerOptions: {
+            ...msg.providerOptions,
+            [sdk]: {
+              ...msg.providerOptions?.[sdk],
+              reasoning_content: "",
+            },
+          },
         }
       }
       return msg