Store message to session

Supports JSON and multipart/form-data. In multipart mode: the payload is a JSON string placed in a form field. The format parameter indicates the format of the input message (default: openai, same as GET). The blob field should be a complete message object: for openai, use OpenAI ChatCompletionMessageParam format (with role and content); for anthropic, use Anthropic MessageParam format (with role and content); for acontext (internal), use {role, parts} format. The optional meta field allows attaching user-provided metadata to the message, which can be retrieved via get_messages().metas or updated via patch_message_meta().