[Rust Frontend] Add Hunyuan A13B tool parser

[Sunbeam23333]

Purpose

The Rust frontend exposes parser selection and automatic model matching, but it did not provide the Python frontend's hunyuan_a13b tool parser. Serving tencent/Hunyuan-A13B-Instruct through the Rust frontend therefore could not parse the model's <tool_calls>[...]</tool_calls> JSON-array format into OpenAI tool-call deltas.

This PR adds a Hunyuan A13B parser using the existing shared incremental JSON tool-call core. It supports parallel calls, empty call arrays, nested argument objects, non-ASCII text, arbitrary streaming chunk boundaries, whitespace between <tool_calls> and the JSON array, and argument properties named name without treating them as new call headers. To match the Python parser, it also strips the leading chat-template 助手： prefix from natural-language responses and discards text after a completed tool-call block, including an empty block. The parser is registered under the Python-compatible hunyuan_a13b name, with model-name matching for the hyphenated and underscored Hunyuan A13B forms.

This contributes to the Rust frontend parity roadmap in #44280.

Duplicate-work check: I reviewed #44280's claims and searched current open PRs for hunyuan rust, hunyuan_a13b, Hunyuan A13B Rust Frontend, HunyuanA13BToolParser, and tool_calls JSON array Rust Frontend. No open PR adds this parser. Related PRs cover HY-V3 reasoning (#48800), Python Hunyuan parser fixes (#49535 and #47954), and a Python Hermes parser-engine migration (#51937), so they do not overlap this Rust implementation.

AI assistance was used to research, implement, test, and review this change. The commit includes the required attribution trailer. This is intentionally a draft so the account owner can complete the final human review before marking it ready.

Test Plan

cargo fmt --all --check
cargo nextest run -p vllm-parser
cargo nextest run -p vllm-chat factory_new_resolves_default_patterns
cargo nextest run -p vllm-chat validate_parser_overrides_rejects_unknown_tool_parser
cargo clippy -p vllm-parser -p vllm-chat --all-targets -- -D warnings
git diff --check origin/main...HEAD

The parser unit tests cover:

• ordinary text without a tool call;
• chat-template prefix removal across streaming chunk boundaries;
• preservation of a non-leading 助手： occurrence in ordinary content;
• parallel calls and deeply nested arguments;
• prefix text and non-ASCII arguments;
• whitespace and newlines before the JSON array;
• empty arrays in complete and character-by-character streaming input;
• discarded suffix text after complete and empty tool-call blocks;
• rejection of an invalid trailing comma;
• arbitrary streaming chunk boundaries;
• nested argument keys named name;
• a truncated tool call failing closed.

Test Result

• cargo fmt --all --check: passed
• Full vllm-parser nextest suite: 431 passed (including 15 Hunyuan tests)
• factory model-pattern test: 1 passed
• unknown-parser registry snapshot test: 1 passed
• Clippy with warnings denied for vllm-parser and vllm-chat: passed
• git diff --check: passed

Clippy emitted only a dependency-level future-incompatibility notice for proc-macro-error2; it emitted no lint failure for this change. An additional full vllm-chat run passed 268 tests before eight network-transport integration tests hit the local sandbox's Operation not permitted restriction and fail-fast cancelled the remainder; the two chat tests affected by this change passed independently. No model evaluation is required because this adds frontend parsing parity and does not change model inference or generated tokens.

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Essential Elements of an Effective PR Description Checklist

• Purpose and related roadmap are documented.
• Exact test commands are provided.
• Test results are reported.
• No model documentation update is required; this adds a frontend parser for an already supported model/parser format.

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Sunbeam23333.

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