mtmd : add const in various places (#28307)

* mtmd : mark context as const in more methods

Mark `mtmd_context` as `const` in:
- mtmd_bitmap_init_lazy
- mtmd_tokenize
- mtmd_tokenize_from_parts
- mtmd_helper_support_video
- mtmd_helper_bitmap_init_from_file
- mtmd_helper_bitmap_init_from_buf
- mtmd_helper_video_init
- mtmd_helper_video_init_from_buf
- mtmd_helper_model_can_chat

The tokenization functions in particular are useful to have marked
`const`, as that allows more easily telling the compiler that we can
safely tokenize from multiple threads (`mtmd_tokenize` is already
documented as thread-safe, this just reifies that in the signature).

* mtmd : mark tokenization input pointer as const

Mark the `bitmaps` and `parts` pointers in `mtmd_tokenize` and
`mtmd_tokenize_from_parts` as `const`. This allows more easily calling
these with immutable arrays / vectors.

* mtmd : mark llama_context as const in mtmd_helper_model_can_chat

tools/mtmd/mtmd.h

@@ -211,7 +211,7 @@ typedef int(* mtmd_bitmap_lazy_callback)(
     mtmd_bitmap ** out_bitmap,
     char ** out_text);
 
-MTMD_API mtmd_bitmap * mtmd_bitmap_init_lazy(mtmd_context * ctx,
+MTMD_API mtmd_bitmap * mtmd_bitmap_init_lazy(const mtmd_context * ctx,
                                              const char * id, // usually set to file hash
                                              void * user_data,
                                              mtmd_bitmap_lazy_callback callback);
@@ -299,10 +299,10 @@ MTMD_API struct mtmd_decoder_pos mtmd_image_tokens_get_decoder_pos(const mtmd_im
 //   0 on success
 //   1 on number of bitmaps not matching the number of markers
 //   2 on media preprocessing error
-MTMD_API int32_t mtmd_tokenize(mtmd_context * ctx,
+MTMD_API int32_t mtmd_tokenize(const mtmd_context * ctx,
                                mtmd_input_chunks * output,
                                const mtmd_input_text * text,
-                               const mtmd_bitmap ** bitmaps,
+                               const mtmd_bitmap * const * bitmaps,
                                size_t n_bitmaps);
 
 // same as mtmd_tokenize(), but takes an array of mtmd_input_part
@@ -311,9 +311,9 @@ MTMD_API int32_t mtmd_tokenize(mtmd_context * ctx,
 // - when you want to control parse_special for each text part
 // note: per-part add_special will be ignored
 // return 1 if a part has both text and bitmap set (or neither)
-MTMD_API int32_t mtmd_tokenize_from_parts(mtmd_context * ctx,
+MTMD_API int32_t mtmd_tokenize_from_parts(const mtmd_context * ctx,
                                           mtmd_input_chunks * output,
-                                          const mtmd_input_part ** parts,
+                                          const mtmd_input_part * const * parts,
                                           size_t n_parts,
                                           bool add_special);