feat(reasoning): honor per-request reasoning_effort on chat completions (#10082)

The OpenAI `reasoning_effort` field only reached the prompt template; it
never toggled the backend's thinking. Map it onto
ReasoningConfig.DisableReasoning (which becomes the enable_thinking gRPC
metadata) in the request merge, so reasoning_effort="none" disables
reasoning per request: the use case from #10072 (run a single Qwen3-style
model and turn reasoning off for low-latency tasks while keeping it on
for others).

Effort levels (minimal/low/medium/high) enable thinking unless the model
config explicitly disabled it (reasoning.disable: true wins and is never
re-enabled by a request); "none" always disables.

Closes #10072


Assisted-by: Claude:claude-opus-4-8 [Claude Code]

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