fix(middleware): parse OpenAI-spec tool_choice in /v1/chat/completions

Follows up on #9526 (the 3-site setter fix) by addressing the remaining
clause in #9508 — string mode and OpenAI-spec specific-function shape both
silently failed in the /v1/chat/completions parsing path.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

Author: Anai-Guo