fix(tools): read text parts from OpenAI array message content

[su6u]

Summary

OpenAI chat content can be a string or an array of parts (text, image, etc). The middleware only treated strings as a real user message, so anything like:

[{ type: "text", text: "I prefer bun" }, { type: "image_url", ... }]

came through as empty.

That meant query/full skipped memory search, and addMemory: "always" skipped saving the turn. String-only chats were fine.

Same file already unwraps text parts when posting to /v4/conversations. Vercel and VoltAgent already do this too. This just does the same for the OpenAI chat path.

Test plan

• unit tests in packages/tools/src/openai/middleware.test.ts (string, array+image, multiple text parts, image-only, save path)
• biome on the two changed files

No breaking changes.