fix(openai-completions): guard string assistant content in transform + tool-history (#93681)

When an assistant message's `content` is a raw string at runtime (JSONL
transcript replay passes it through even though the type declares an array),
the OpenAI-compatible completions path crashes:

- `transformMessages` called `assistantMsg.content.flatMap(...)` ->
  `TypeError: ... .flatMap is not a function` (first crash, always hit).
- Two `hasToolHistory` helpers (`openai-transport-stream.ts` and
  `openai-completions.ts`) called `content.some(...)` -> `TypeError: ...
  .some is not a function` (siblings, surface once the flatMap crash is fixed).

Normalize a string assistant content to an equivalent single text block
before transforming (matching the string->text handling already used in
anthropic-payload-policy.ts), and `Array.isArray`-guard both `hasToolHistory`
helpers so a string assistant simply does not count toward tool history.

Verified end-to-end through the real `buildOpenAICompletionsParams` and
`streamOpenAICompletions` entry points: before the fix a string-content
assistant followed by a toolResult throws TypeError; after the fix params are
produced correctly (string preserved as text, tool history detected). Normal
array content is unaffected.

src/agents/openai-transport-stream.ts

@@ -2524,7 +2524,11 @@ function hasToolHistory(messages: Context["messages"]): boolean {
   return messages.some(
     (message) =>
       message.role === "toolResult" ||
-      (message.role === "assistant" && message.content.some((block) => block.type === "toolCall")),
+      // Assistant content can be a raw string from transcript replay; a string
+      // never carries tool calls, so it should not count toward tool history.
+      (message.role === "assistant" &&
+        Array.isArray(message.content) &&
+        message.content.some((block) => block.type === "toolCall")),
   );
 }
 

src/llm/providers/openai-completions.ts

@@ -63,7 +63,9 @@ function hasToolHistory(messages: Message[]): boolean {
       return true;
     }
     if (msg.role === "assistant") {
-      if (msg.content.some((block) => block.type === "toolCall")) {
+      // Assistant content can be a raw string from transcript replay; a string
+      // never carries tool calls, so it should not count toward tool history.
+      if (Array.isArray(msg.content) && msg.content.some((block) => block.type === "toolCall")) {
         return true;
       }
     }

src/llm/providers/transform-messages.ts

@@ -117,7 +117,15 @@ export function transformMessages<TApi extends Api>(
           assistantMsg.api === model.api &&
           assistantMsg.model === model.id);
 
-      const transformedContent = assistantMsg.content.flatMap((block) => {
+      // Assistant content is typed as a block array, but transcript replay can
+      // hand us a raw string (JSONL passthrough). Normalize it to an equivalent
+      // single text block before transforming, matching the string->text block
+      // handling already used in anthropic-payload-policy.ts.
+      const contentBlocks = Array.isArray(assistantMsg.content)
+        ? assistantMsg.content
+        : [{ type: "text" as const, text: assistantMsg.content as unknown as string }];
+
+      const transformedContent = contentBlocks.flatMap((block) => {
         if (block.type === "thinking") {
           if (modelBoundThinkingReplayMode === "drop") {
             return [];