fix(chat-completions): accept object content parts and absent content field from WorkBuddy

[Windyztree]

Problem

WorkBuddy's custom models (e.g. when proxying claude-opus-5) serialize a
message's content into shapes that fall outside the OpenAI spec, which only
allows content to be a string, an array of parts, or null. The proxy's
Zod validation then rejects the request:

Request validation failed: messages.N.content: Invalid input

Two distinct shapes trigger this:

1. Single content-part object — content: { type: 'text', text: '...' }
   instead of the spec-required string / array.
2. Absent content field — an assistant turn that carries tool_calls
   but omits content entirely (undefined). OpenAI treats this as null.

Both appear in real multi-turn agentic tool-call conversations, and the error
only surfaces once the conversation accumulates enough messages that the
offending shape reaches the validator.

Fix

• Extend ChatCompletionsPayloadSchema to also accept a single content-part
  object and an absent/undefined content field.
• Normalize in normalizeChatCompletionContent before forwarding (structure
  only, semantics unchanged):
  • { type: 'text', text } → plain string text
  • any other single object → one-element array [object]
  • absent content → null

Changes

• src/lib/schemas.ts — content union adds z.undefined() and .optional()
• src/lib/chat-completions-content.ts — normalize absent content to null
• src/routes/chat-completions/handler.ts — call normalizer after validation
• tests/chat-completions-content-normalization.test.ts — unit + e2e for both
  shapes (9 tests, all passing)

Test plan

• bun test tests/chat-completions-content-normalization.test.ts — 9 pass
• Live check against a real claude-opus-5 multi-turn tool-call session:
  Invalid input no longer occurs; completions return 200.

Key diff

src/lib/schemas.ts

+    // WorkBuddy custom models also emit assistant messages with `tool_calls`
+    // but NO `content` field. OpenAI treats that as null, so accept it.
     content: z.union([
       z.string(),
       z.array(z.unknown()),
       z.record(z.string(), z.unknown()),
       z.null(),
+      z.undefined(),
-    ]),
+    ]).optional(),

src/lib/chat-completions-content.ts

 function normalizeMessageContent(message: Message): Message {
   const content = message.content as unknown

+  // WorkBuddy: assistant with tool_calls but no content -> treat as null
+  if (content === undefined)
+    return { ...message, content: null }
+
   if (content === null || typeof content === 'string' || Array.isArray(content))
     return message