Fix chat template: render assistant tool_calls

Assistant messages carrying `tool_calls` render to nothing under the current
template: the message loop ends in a single `{{- content + "<|im_end|>\n" -}}`
with no branch for tool calls. Two consequences, both reproduced on
llama.cpp `server-rocm-b10088`:

1. llama.cpp probes the template at startup, sees that a tool call renders to
nothing, and reports `supports_tool_calls: false` /
`supports_parallel_tool_calls: false` in `/props`. It then constrains
generation to at most one call, so the second call of a parallel request is
never sampled.
2. Replaying a conversation that already contains an assistant tool call
produces an empty assistant turn, while the following `tool` result is still
rendered -- the model sees an answer to a question it has no record of asking.

**This PR** adds the `format_arg_value` / `render_tool_calls` macros (taken
verbatim from the latest LFM2.5 template) and one branch that emits them, so a
tool call renders as
`<|tool_call_start|>[name(arg='value'), ...]<|tool_call_end|>`.
It also changes `message["content"]` to `message.get("content")`: an assistant
message with `tool_calls` and no `content` key -- the ordinary OpenAI-style
payload -- otherwise raises
`TypeError: Object of type Undefined is not JSON serializable`.

Nothing else changes. The tool-marker convention, thinking handling and
formatting are untouched, and a conversation without tools renders
byte-identically to the current template.

**Verified** on llama.cpp: `chat_template_caps` goes false -> true, parallel
tool calls 1 -> 2, and the replayed assistant turn is preserved. transformers
and minja render identically. Benchmark runs (IFBench, MATH-500, GPQA Diamond,
AA-Omniscience, BFCL v4 multiple/multi_turn_base/irrelevance) are in flight;
please hold merge until those confirm no regression.

chat_template.jinja

@@ -1,4 +1,31 @@
 {{- bos_token -}}
+{%- macro format_arg_value(arg_value) -%}
+	{%- if arg_value is string -%}
+		{{- "'" + (arg_value | replace("\\", "\\\\") | replace("'", "\\'") | replace("\n", "\\n") | replace("\r", "\\r")) + "'" -}}
+	{%- elif arg_value is mapping or arg_value is iterable -%}
+		{{- arg_value | tojson -}}
+	{%- else -%}
+		{{- arg_value | string -}}
+	{%- endif -%}
+{%- endmacro -%}
+{%- macro render_tool_calls(tool_calls) -%}
+	{%- set tool_calls_ns = namespace(tool_calls=[]) -%}
+	{%- for tool_call in tool_calls -%}
+		{%- set func = tool_call["function"] if "function" in tool_call else tool_call -%}
+		{%- set func_name = func["name"] -%}
+		{%- set func_args = func.get("arguments") -%}
+		{%- set args_ns = namespace(arg_strings=[]) -%}
+		{%- if func_args is mapping -%}
+			{%- for arg_name, arg_value in func_args.items() -%}
+				{%- set args_ns.arg_strings = args_ns.arg_strings + [arg_name + "=" + format_arg_value(arg_value)] -%}
+			{%- endfor -%}
+		{%- elif func_args is string and (func_args | trim) not in ["", "{}", "null"] -%}
+			{{- raise_exception("Tool call arguments must be a mapping, got a JSON-encoded string: parse arguments with json.loads() before applying the chat template") -}}
+		{%- endif -%}
+		{%- set tool_calls_ns.tool_calls = tool_calls_ns.tool_calls + [func_name + "(" + (args_ns.arg_strings | join(", ")) + ")"] -%}
+	{%- endfor -%}
+	{{- "<|tool_call_start|>[" + (tool_calls_ns.tool_calls | join(", ")) + "]<|tool_call_end|>" -}}
+{%- endmacro -%}
 {%- set system_prompt = "" -%}
 {%- set ns = namespace(system_prompt="") -%}
 {%- if messages[0]["role"] == "system" -%}
@@ -23,14 +50,22 @@
 {%- endif -%}
 {%- for message in messages -%}
 	{{- "<|im_start|>" + message["role"] + "\n" -}}
-	{%- set content = message["content"] -%}
+	{%- set content = message.get("content") -%}
 	{%- if content is not string -%}
 		{%- set content = content | tojson -%}
 	{%- endif -%}
 	{%- if message["role"] == "tool" -%}
 		{%- set content = "<|tool_response_start|>" + content + "<|tool_response_end|>" -%}
 	{%- endif -%}
-	{{- content + "<|im_end|>\n" -}}
+	{%- if message["role"] == "assistant" and message.get("tool_calls") -%}
+		{%- if content and content != "null" -%}
+			{{- content -}}
+		{%- endif -%}
+		{{- render_tool_calls(message["tool_calls"]) -}}
+		{{- "<|im_end|>\n" -}}
+	{%- else -%}
+		{{- content + "<|im_end|>\n" -}}
+	{%- endif -%}
 {%- endfor -%}
 {%- if add_generation_prompt -%}
 	{{- "<|im_start|>assistant\n" -}}