* Fix review findings for PR #49
1. Sandbox fallback Jinja env in _VariantTokenizerProxy.apply_chat_template
(use SandboxedEnvironment, matching _derive_assistant_prefix_by_render)
2. Unwrap benign outer-If guards in _template_ends_with_toplevel_for so
templates like {% if messages %}{% for ... %}{% endfor %}{% endif %}
are still repairable (preserves Qwen3-Guard rejection via else-branch
and add_generation_prompt-name checks)
3. Preserve raw name_or_path in _VariantTokenizerProxy._source_path so
local-path detection works for dict/list variant tokenizers
4. Context-aware strict-mode messages: omit "will still load" and
"Set UNSLOTH_STRICT_CHAT_TEMPLATE=1" when already raising
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