fix: properly terminate llama.cpp kv_overrides array with empty key + updated doc (#6672)

* fix: properly terminate kv_overrides array with empty key

The llama model loading function expects KV overrides to be terminated
with an empty key (key[0] == 0). Previously, the kv_overrides vector was
not being properly terminated, causing an assertion failure.

This commit ensures that after parsing all KV override strings, we add a
final terminating entry with an empty key to satisfy the C-style array
termination requirement. This fixes the assertion error and allows the
model to load correctly with custom KV overrides.

Fixes #6643

- Also included a reference to the usage of the `overrides` option in
  the advanced-usage section.

Signed-off-by: blob42 <contact@blob42.xyz>

* doc: document the `overrides` option

---------

Signed-off-by: blob42 <contact@blob42.xyz>

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

@@ -291,6 +291,11 @@ static void params_parse(server_context& ctx_server, const backend::ModelOptions
         }
     }
 
+    if (!params.kv_overrides.empty()) {
+        params.kv_overrides.emplace_back();
+        params.kv_overrides.back().key[0] = 0;
+    }
+
     // TODO: Add yarn
 
     if (!request->tensorsplit().empty()) {