fix: replace empty text in reasoning messages to preserve positions

normalizeMessages removes empty text parts, which shifts thinking block
positions and invalidates signatures. Simple preservation does not work
because the AI SDK has a second filter and the API rejects empty text.

In assistant messages with signed reasoning, replace empty text with a
placeholder instead of removing it. This preserves array positions through
all filtering layers.

Author: chan1103

packages/opencode/src/provider/transform.ts

@@ -61,13 +61,27 @@ export namespace ProviderTransform {
             return msg
           }
           if (!Array.isArray(msg.content)) return msg
-          const filtered = msg.content.filter((part) => {
-            if (part.type === "text" || part.type === "reasoning") {
-              return part.text !== ""
-            }
-            return true
-          })
+          const hasReasoning = msg.role === "assistant" && msg.content.some((p) => p.type === "reasoning" && p.providerOptions !== undefined)
+          const filtered = msg.content
+            .filter((part) => {
+              if (part.type === "reasoning") {
+                return part.text !== "" || part.providerOptions !== undefined
+              }
+              if (part.type === "text" && !hasReasoning) {
+                return part.text !== ""
+              }
+              return true
+            })
+            .map((part) => {
+              if (hasReasoning && part.type === "text" && part.text === "") {
+                return { ...part, text: "..." } as typeof part
+              }
+              return part
+            })
           if (filtered.length === 0) return undefined
+          if (hasReasoning && filtered.length > 0 && filtered[filtered.length - 1].type === "reasoning") {
+            filtered.push({ type: "text", text: "..." } as (typeof filtered)[number])
+          }
           return { ...msg, content: filtered }
         })
         .filter((msg): msg is ModelMessage => msg !== undefined && msg.content !== "")