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/Cargo.toml

@@ -21,3 +21,4 @@ tokio.workspace = true
 axum.workspace = true
 uuid.workspace = true
 tokio-stream.workspace = true
+minijinja.workspace = true