fix(schemas): ChatMessage.content accepts str | list for multimodal

OpenAI ChatMessage content can be either a string (simple text) or a list
of content parts (multimodal: text + image_url, etc.). Updated the type
annotation to match the actual API. No behavioral change; this is a
type-hint-only widening so callers can pass multimodal content via
ChatMessage instead of dicts.

Required by tests/test_openai_compatible.py::test_vision_multimodal_message
which was passing raw dicts to OpenAICompatibleRequest (wrong - the field
is typed list[ChatMessage]). With this widening, that test can now use
ChatMessage(role='user', content=[...multimodal parts]) without losing
type fidelity.

src/openai_schemas.py

@@ -48,7 +48,7 @@ class ToolCall:
 @dataclass(frozen=True)
 class ChatMessage:
  role: str
- content: str
+ content: str | list  # str for text; list of content parts for multimodal (text + image_url, etc.)
  tool_calls: Optional[tuple[ToolCall, ...]] = None
  tool_call_id: Optional[str] = None
  name: Optional[str] = None