Add generation markers to the chat template

This PR adds `{% generation %}` / `{% endgeneration %}` markers around the assistant output in `chat_template.jinja`, so `return_assistant_tokens_mask=True` returns the assistant tokens. Without them the mask is all zeros, and assistant-only loss (e.g. TRL SFT with `assistant_only_loss=True`) can't work. Rendering is unchanged.

The `<|im_start|>assistant\n` header is now emitted once before the branches, so it stays outside the mask. The mask covers what the model generates, up to and including `<|im_end|>`:

```
<think>\nr\n</think>\n\nHello<|im_end|>\n
```

Checked: the rendered prompt is identical before and after for 40 combinations (system, multi-turn, tool calls and responses, with and without `tools`, `add_generation_prompt`, `enable_thinking`). The mask covers each assistant turn in multi-turn and tool-call conversations, and the `<|im_end|>` stop token is always inside it.

chat_template.jinja

@@ -40,14 +40,16 @@
                 {%- set content = content.split('</think>')[-1].lstrip('\n') %}
             {%- endif %}
         {%- endif %}
+        {{- '<|im_start|>' + message.role + '\n' }}
+        {%- generation %}
         {%- if loop.index0 > ns.last_query_index %}
             {%- if loop.last or (not loop.last and reasoning_content) %}
-                {{- '<|im_start|>' + message.role + '\n<think>\n' + reasoning_content.strip('\n') + '\n</think>\n\n' + content.lstrip('\n') }}
+                {{- '<think>\n' + reasoning_content.strip('\n') + '\n</think>\n\n' + content.lstrip('\n') }}
             {%- else %}
-                {{- '<|im_start|>' + message.role + '\n' + content }}
+                {{- content }}
             {%- endif %}
         {%- else %}
-            {{- '<|im_start|>' + message.role + '\n' + content }}
+            {{- content }}
         {%- endif %}
         {%- if message.tool_calls %}
             {%- for tool_call in message.tool_calls %}
@@ -69,6 +71,7 @@
             {%- endfor %}
         {%- endif %}
         {{- '<|im_end|>\n' }}
+        {%- endgeneration %}
     {%- elif message.role == "tool" %}
         {%- if loop.first or (messages[loop.index0 - 1].role != "tool") %}
             {{- '<|im_start|>user' }}