[Rust Frontend] Fix macro-based content format detection

[reidliu41]

Purpose

The Rust chat template detector only recognized loops directly over
message.content. It missed templates that pass the content into a macro and
iterate with {% for item in content %}.

Qwen3.5 uses the macro form, causing auto mode to incorrectly select the
string content format. Multimodal messages consequently lost the
<|vision_start|> and <|vision_end|> wrappers around <|image_pad|>, which
could produce incorrect prompt tokens and multimodal positions.

Recognize loops over a content parameter while preserving the existing
direct message.content detection. Add regression coverage using both a
minimal macro template and the repository's Qwen3.5 template.

The corrected auto-detection matches the explicit OpenAI content format and
the corresponding Python detector behavior.

Test Plan

  cargo nextest run \
    -p vllm-chat \
    -E 'test(detects_openai_template_with_content_parameter_loop) | test(qwen35_template_auto_detects_openai_multimodal_content)'

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

[claude]

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