[Rust Frontend] Extend renderer/parser roundtrip tests to support token ids

[BugenZhao]

Purpose

Some model paths are token-id native: renderers may directly yield token ids, and output processors may directly consume token ids. This PR extends the roundtrip tests to support raw token ids in those cases, demonstrated by the GPT-OSS/Harmony path.

Test Plan

cargo fmt --all
cargo nextest run -p vllm-chat roundtrip_gpt_oss
cargo nextest run -p vllm-chat roundtrip_qwen3_reasoning_and_content
git diff --check

Test Result

All passed locally. I also verified that roundtrip_gpt_oss fails if generated chunk token ids are cleared, which confirms the test covers the token-id output path.

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[BugenZhao]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Take the Harmony cache lock for GPT-OSS roundtrip

This new GPT-OSS case loads the Harmony renderer/output path, which calls load_harmony_encoding; the Harmony output tests document that tests loading this encoding need the shared harmony_tiktoken_cache file lock so cargo nextest cannot populate the upstream tiktoken cache concurrently (rust/src/chat/src/output/harmony/tests.rs:1-4). Because this integration test only takes the per-model hf_gpt_oss lock, a full cargo nextest run -p vllm-chat can run it in parallel with the locked Harmony tests and reintroduce that cache race; add the same #[file_serial(harmony_tiktoken_cache)] attribute to this fixture.

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[BugenZhao]

Good catch but not correct fix direction. I just realized after #46799 we don't even need existing harmony_tiktoken_cache anymore, since it embeds necessary vocab files directly into the binary and won't download at all.