[NPUW] Fix model builder hidden_size defaults

Defer norm/ffn defaults in ModelConfig to build_model() so changing
hidden_size after construction takes effect. Previously the constructor
baked in defaults using the initial hidden_size, silently ignoring
later changes.

Author: dylanneve1

src/plugins/intel_npu/tests/functional/behavior/npuw/test_engine/models/model_builder.cpp

@@ -1392,10 +1392,20 @@ std::shared_ptr<ov::Model> ModelBuilder::make_model(const ov::Output<ov::Node>&
     return std::make_shared<ov::Model>(ov::OutputVector{res->output(0)}, m_sinks, model_name);
 }
 
-std::shared_ptr<ov::Model> ModelBuilder::build_model(const ModelConfig& config) {
+std::shared_ptr<ov::Model> ModelBuilder::build_model(const ModelConfig& config_in) {
     OPENVINO_ASSERT(
-        (int)config.use_conv_features + (int)config.use_cross_attention + (int)config.use_token_type_embedding <= 1,
+        (int)config_in.use_conv_features + (int)config_in.use_cross_attention + (int)config_in.use_token_type_embedding <= 1,
         "At most one structural dispatch flag may be set");
+
+    // Fill in norm/ffn defaults from actual config sizes when the caller left them empty.
+    ModelConfig config = config_in;
+    if (!config.norm) {
+        config.norm = LayerNorm(config.hidden_size, config.precision);
+    }
+    if (!config.ffn) {
+        config.ffn = SwiGLU(config.hidden_size, config.intermediate_size, config.precision, config.weight);
+    }
+
     if (config.use_conv_features) {
         return build_whisper_encoder(config);
     }

src/plugins/intel_npu/tests/functional/behavior/npuw/test_engine/models/model_builder.hpp

@@ -411,10 +411,7 @@ struct ModelConfig {
     size_t max_position_embeddings = 512;
     size_t type_vocab_size = 2;
 
-    ModelConfig()
-        : lm_head_weight(weight),
-          norm(LayerNorm(hidden_size, precision)),
-          ffn(SwiGLU(hidden_size, intermediate_size, precision, weight)) {}
+    ModelConfig() : lm_head_weight(weight) {}
 
     size_t get_kv_heads() const {
         return num_kv_heads == 0 ? num_heads : num_kv_heads;