mtmd, server: add "placeholder bitmap" for counting tokens , add */input_tokens API (#23913)

* mtmd: add "placeholder bitmap" for counting tokens w/o preprocessing

* fast path skip preproc for placeholder

* fix build

* correct the api

* add server endpoint + tests

* add object name

* update docs

* add proxy handling

* fix build

* fix audio input path

* use is_placeholder in process_mtmd_prompt()

* nits

* nits (2)

* docs: clarify chat/completions/input_tokens is not official

* fix merge problem

tools/mtmd/mtmd.h

@@ -136,6 +136,11 @@ MTMD_API int mtmd_get_audio_sample_rate(const mtmd_context * ctx);
 // if bitmap is audio:
 //     length of data must be n_samples * sizeof(float)
 //     the data is in float format (PCM F32)
+//
+// if data == nullptr:
+//     the bitmap is considered "empty", and will be treated as a placeholder for counting tokens
+//     you can pass the bitmap via mtmd_tokenize(), then call mtmd_*_get_n_tokens() to count the tokens
+//     note: passing a placeholder bitmap to mtmd_encode() will return an error
 MTMD_API mtmd_bitmap *         mtmd_bitmap_init           (uint32_t nx, uint32_t ny, const unsigned char * data);
 MTMD_API mtmd_bitmap *         mtmd_bitmap_init_from_audio(size_t n_samples,         const float         * data);
 MTMD_API uint32_t              mtmd_bitmap_get_nx     (const mtmd_bitmap * bitmap);