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/models/granite-speech.cpp

@@ -1,7 +1,7 @@
 #include "models.h"
 
 ggml_cgraph * clip_graph_granite_speech::build() {
-    const int n_frames     = img.nx;
+    const int n_frames     = img.nx();
     const int context_size = hparams.audio_chunk_size;
     const int ctc_layer    = n_layer / 2;
     const int conv_kernel  = hparams.audio_conv_kernel_size;