bug : WebUI regenerate breaks message order

[ggerganov]

Happens with certain models such as Gemma3:

Repro:

make -j && ./bin/llama-server -hf ggml-org/gemma-3-1b-it-qat-GGUF --jinja

• Start new conv
• Type "Hello"
• Wait for response to finish
• Click regenerate
• Wait for response to finish
• Type "Test"
• Observe error

[ggerganov]

Looking more into it, I think it happens for all models. It's just that only Gemma's template detects the incorrect order of the roles and throws an error.

[ddh0]

cc @allozaur :)

(Oops, I see he was already assigned. My bad.)