fix(neuron): render HF chat templates via minijinja pycompat

The Qwen3.6 chat_template.jinja (now loaded after the precedence fix)
failed to render in minijinja: it uses Python str methods
(content.startswith/endswith/split/rstrip/lstrip) and the raise_exception
global that HF transformers patches into its Jinja env but minijinja
doesn't provide. The render error tripped the text-only fallback, so
image requests still produced zero <|image_pad|> tokens.

Wire the standard bridge into render_chat_template:
- minijinja-contrib `pycompat::unknown_method_callback` supplies the
  Python string/list/dict methods;
- a `raise_exception` global maps to a render error (so malformed inputs
  — e.g. an image in a system message — surface cleanly).

Add the real Qwen3.6-27B chat_template.jinja (verbatim from beast's HF
cache) as a test fixture and assert it renders one <|image_pad|> for a
text+image turn — the end-to-end check that would have caught this
before deploy.

Refs #16 / TP-vision.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -2508,6 +2508,16 @@ dependencies = [
  "serde_json",
 ]
 
+[[package]]
+name = "minijinja-contrib"
+version = "2.20.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "99df5123c54391e2a228014c1dbbd85a3dab08a25e776c810526f2f47542b3de"
+dependencies = [
+ "minijinja",
+ "serde",
+]
+
 [[package]]
 name = "minimal-lexical"
 version = "0.2.1"
@@ -2605,6 +2615,7 @@ dependencies = [
  "hf-hub",
  "image",
  "minijinja",
+ "minijinja-contrib",
  "reqwest",
  "safetensors 0.7.0",
  "serde",