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.

examples/common-whisper.h

@@ -14,6 +14,14 @@ bool read_audio_data(
         std::vector<std::vector<float>> & pcmf32s,
         bool stereo);
 
+// decode audio bytes already held in memory (uploaded file, network buffer)
+bool read_audio_data(
+        const char * buffer,
+        size_t buffer_size,
+        std::vector<float> & pcmf32,
+        std::vector<std::vector<float>> & pcmf32s,
+        bool stereo);
+
 // convert timestamp to string, 6000 -> 01:00.000
 std::string to_timestamp(int64_t t, bool comma = false);