server : return language in detect response

[turicas]

When detect_language=true is used with response_format=json, the server detects the language but returns only an empty text field.

Before this change:

curl -s http://127.0.0.1:8080/inference \
  -F file=@/work/audio.wav \
  -F response_format=json \
  -F detect_language=true \
  -F language=auto

{"text":""}

After this change:

{"text":"","language":"portuguese"}

It uses whisper_full_lang_id(ctx).

Tested with ggml-tiny.bin. Also tested:

cmake --build build --target whisper-server -j2
ctest --output-on-failure -R 'test-whisper-zero-samples|test-whisper-buffer-loader'

Resolves: #3603

[turicas]

Thanks, @danbev!