SerializeMessages is the wire serializer shared by every OpenAI-style
backend in the project (openai_compat, azure, deepseek, glm, qwen,
groq, and other endpoints that accept the OpenAI Chat Completions
schema). The OpenAI spec defines an optional `name` field on each
message that disambiguates participants in multi-user conversations.
This change adds the field to the openaiMessage wire struct (with
omitempty) and the multipart map branch (only when set), so any
Message.Name produced upstream is forwarded to the model. Sanitization
upstream guarantees the wire-level constraint ^[a-zA-Z0-9_-]{1,64}$,
so backends that strictly enforce it will not reject the request.
Tests cover three properties:
- name appears on the plain-text path when set
- name appears on the multipart-with-media path when set
- both paths omit the field entirely when Name is empty (legacy
single-user channels stay byte-identical on the wire)
Refs #2702.