fix: send content: null instead of content: "" for tool-only assistant messages

[robert-j-y]

Description

Assistant messages containing only tool calls (no text) were serialized with content: "". Providers that strictly validate the OpenAI chat format (e.g. AWS Bedrock / Nova) reject blank content with a 400 error:

The text field in the ContentBlock object at messages.X.content.0 is blank. Add text to the text field, and try again.

Root cause: In convertToOpenRouterChatMessages, text is initialized as "" and never changes when the assistant turn has only tool-call parts. The type already allows string | null.

Fix: content: text → content: text || null — one-line change that coerces empty string to null, which the OpenAI spec explicitly allows for assistant messages. Non-empty text is unaffected.

Before:

messages.push({
  role: 'assistant',
  content: text,           // "" for tool-only turns → Bedrock 400
  tool_calls: toolCalls.length > 0 ? toolCalls : undefined,
});

After:

messages.push({
  role: 'assistant',
  content: text || null,   // null for tool-only turns → accepted
  tool_calls: toolCalls.length > 0 ? toolCalls : undefined,
});

Same root cause Vercel fixed in @ai-sdk/openai-compatible (vercel/ai#13466).

Checklist

• I have run pnpm stylecheck and pnpm typecheck
• I have run pnpm test and all tests pass
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)

Changeset

• I have run pnpm changeset to create a changeset file

Note: A changeset is required for your changes to trigger a release. If your PR only contains docs, tests, or CI changes that don't need a release, run pnpm changeset --empty instead.