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/conformer.cpp

@@ -1,7 +1,7 @@
 #include "models.h"
 
 ggml_cgraph * clip_graph_conformer::build() {
-    const int n_frames   = img.nx;
+    const int n_frames   = img.nx();
     const int n_pos      = n_frames / 2;
     const int n_pos_embd = (((((n_frames + 1) / 2) + 1) / 2 + 1) / 2) * 2 - 1;
     GGML_ASSERT(model.position_embeddings->ne[1] >= n_pos);