FIX: mtmd_tokenize: error (#2360)

DEEPSEEK4 SLOP Explanation. (Tested and is working correctly after build) 

fix mtmd: missing text_len in gpttype_adapter.cpp aggregate init

Commit 4114ba18b ("mtmd: fix silent prompt truncation on embedded NUL
(#25548)") added a `text_len` field to `mtmd_input_text`, changing the
struct layout from:

    { text, add_special, parse_special }

to:

    { text, text_len, add_special, parse_special }

The aggregate initialization in `gpttype_adapter.cpp` was never updated
to account for the new field.  With C++ aggregate init, the positional
arguments shifted:

    mtmd_input_text inp_txt = {
        mtmd_default_marker(),   // text  = "<__media__>"
        /* add_special */ false, // text_len = 0         <-- BUG
        /* parse_special */ true, // add_special = true
    };
    // parse_special is zero-initialized

Because text_len was 0, `input_text.assign(text->text, text->text_len)`
in the mtmd_tokenizer constructor produced an empty string.
`split_text()` on an empty string returned an empty vector, yielding
zero media markers in the text -- but one bitmap was still provided,
triggering the error:

    mtmd_tokenize: error: number of media markers in text (0) does not
    match number of bitmaps (1)

Fix by explicitly capturing the marker pointer and passing its length.

gpttype_adapter.cpp

@@ -5334,8 +5334,10 @@ static void PrepareMediaEmbds(const int nctx, const std::vector<int> & media_int
                 printf("\nError: MTMD media %d failed to load!",i);
                 continue;
             }
+            const auto * marker = mtmd_default_marker();
             mtmd_input_text inp_txt = {
-                mtmd_default_marker(),
+                marker,
+                strlen(marker),
                 /* add_special */ false,
                 /* parse_special */ true,
             };