common : add LLM-jp-4.1 Harmony dialect handler (#29681)

LLM-jp-4.1 uses the GPT-OSS format, but its tokenizer decodes a space
after every special token and parallel tool calls are separated by
<|end|>. The GPT-OSS handler rejects this output, so add a dedicated
handler, selected by the chat_format=llm-jp-harmony-v1 declaration in
the chat template.

Assisted-by: Claude Fable 5.1