server: Jinja chat template rendering via minijinja

Load the model's chat_template.jinja (or tokenizer_config.json
chat_template field) at startup and render it with minijinja instead of
hardcoded per-model prompt builders.

Custom Jinja functions: strftime_now (date formatting), raise_exception
(template validation errors).  Falls back to Qwen3 ChatML template if
no Jinja template is found.

Removes the hardcoded build_prompt_gpt_oss() — the model's own template
now drives prompt formatting, matching llama.cpp's behavior exactly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/xserv-server/src/main.rs

@@ -11,7 +11,7 @@ use xserv_model::ModelConfig;
 
 pub struct AppState {
     pub model_name: String,
-    pub model_type: String,
+    pub chat_template: api::ChatTemplate,
     pub engine_sender: Mutex<mpsc::Sender<GenerateRequest>>,
     pub engine_tokenizer: Mutex<xserv_tokenizer::Tokenizer>,
     pub max_seq_len: usize,
@@ -101,9 +101,10 @@ async fn main() {
     });
 
     let model_type = model_config.model_type.clone().unwrap_or_default();
+    let chat_template = api::ChatTemplate::load(&model_dir, &model_type);
     let state = Arc::new(AppState {
         model_name,
-        model_type,
+        chat_template,
         engine_sender: Mutex::new(tx),
         engine_tokenizer: Mutex::new(tokenizer),
         max_seq_len,