fix(vision): propagate mtmd media marker from backend via ModelMetadata (#9412)

Upstream llama.cpp (PR #21962) switched the server-side mtmd media
marker to a random per-server string and removed the legacy
"<__media__>" backward-compat replacement in mtmd_tokenizer. The
Go layer still emitted the hardcoded "<__media__>", so on the
non-tokenizer-template path the prompt arrived with a marker mtmd
did not recognize and tokenization failed with "number of bitmaps
(1) does not match number of markers (0)".

Report the active media marker via ModelMetadataResponse.media_marker
and substitute the sentinel "<__media__>" with it right before the
gRPC call, after the backend has been loaded and probed. Also skip
the Go-side multimodal templating entirely when UseTokenizerTemplate
is true — llama.cpp's oaicompat_chat_params_parse already injects its
own marker and StringContent is unused in that path. Backends that do
not expose the field keep the legacy "<__media__>" behavior.

backend/cpp/llama-cpp/grpc-server.cpp

@@ -2814,6 +2814,13 @@ public:
             return grpc::Status(grpc::StatusCode::FAILED_PRECONDITION, "Model not loaded");
         }
 
+        // Report the active multimodal media marker so the Go layer can emit the
+        // same string when rendering prompts outside the tokenizer-template path.
+        // Only meaningful when an mtmd context was initialized (vision/audio models).
+        if (ctx_server.impl->mctx != nullptr) {
+            response->set_media_marker(get_media_marker());
+        }
+
         // Check if chat templates are initialized
         if (ctx_server.impl->chat_params.tmpls == nullptr) {
             // If templates are not initialized, we can't detect thinking support