common : fix server /inference fails to decode in-memory audio (regression) (#3818)

common : fix server /inference fails to decode in-memory audio (regression) (#3818)

* common: add memory buffer overload of read_audio_data

whisper-server /inference without --convert passed the uploaded file
bytes to read_audio_data as a filename, so ma_decoder_init_file tried
to open a path starting with "RIFF" and failed. every request returned
HTTP 400 "Invalid request" on builds without WHISPER_FFMPEG, which is
the default.

factor the PCM extraction into a shared helper and add an overload that
decodes straight from a memory buffer via ma_decoder_init_memory, which
the function already used for the stdin path. server now calls it with
the upload content. the filename overload behavior is unchanged.