[Rust Frontend] Add seed_oss and step3p5 reasoning parsers

[yzhan1]

Purpose

Adds two reasoning parsers to the Rust frontend, addressing parser-parity items from the Rust Frontend Feature Parity roadmap (#44280): seed_oss (<seed:think>/</seed:think> delimited; defaults into reasoning so streams that omit the opening delimiter still parse) and step3p5 (standard <think>/</think> plus drop-newline framing around </think>; holds a trailing reasoning \n across pushes until either more reasoning text or </think> arrives, and drops a leading \n from the first content delta after the boundary; defaults into reasoning).

Also teaches the shared DelimitedReasoningParser to skip a literal start token while already in reasoning mode, so default_in_reasoning = true parsers (DeepSeekR1, SeedOSS, Step3p5) now handle streams that do emit an explicit start delimiter — a literal <think> or <seed:think> is treated as a no-op rather than leaking into reasoning text. This brings them to behavioral parity with Python's BaseThinkingReasoningParser for the explicit start-token case, and fixes a pre-existing limitation of DeepSeekR1ReasoningParser that is pinned by a new test.

Adds model-pattern auto-detection: step-3p5 / step3p5 / step-3.5 (placed before the existing step3 pattern so the more specific match wins), and seed-oss / seedoss.
Adds end-to-end roundtrip tests for ByteDance-Seed/Seed-OSS-36B-Instruct and stepfun-ai/Step-3.5-Flash, exercising the real HF tokenizer and chat template through the chat output processor.

Test Plan

cargo nextest run -p vllm-reasoning-parser
cargo nextest run -p vllm-chat
cargo nextest run --workspace --no-fail-fast
cargo clippy -p vllm-reasoning-parser -p vllm-chat --tests
cargo fmt --check

Test Result

vllm-reasoning-parser: 36 passed (15 new unit tests covering single-push, split-push, partial-delimiter, prompt-boundary, unterminated, empty, multi-newline framing, no-start-token compat, explicit-start-token, multi-push streaming, and end-of-stream cases for each parser, plus direct coverage of the shared DelimitedReasoningParser strip-and-buffer behavior).
vllm-chat: 179 passed (including 2 new roundtrip cases). The expect-snapshot for the unknown-parser error message was updated to include the new names; factory tests pin the new patterns.
Workspace: 842 passed, 1 skipped.
cargo clippy and cargo fmt --check: clean.

Duplicate-check evidence (per AGENTS.md): ran gh issue view 44280 --repo vllm-project/vllm --comments, gh pr list --repo vllm-project/vllm --state open --search "44280 in:body", and gh pr list --repo vllm-project/vllm --state all --search "rust frontend reasoning parser". The open roadmap PRs (#44391, #44321, #44382, #44499) target other unrelated items. No open or merged PR adds seed_oss or step3p5 reasoning parsers in Rust.

AI assistance was used in producing this change. Every changed line was reviewed and tested locally on the submitter's machine before opening.

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[claude]

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[BugenZhao]

Also teaches the shared DelimitedReasoningParser to skip a literal start token while already in reasoning mode, so default_in_reasoning = true parsers (DeepSeekR1, SeedOSS, Step3p5) now handle streams that do emit an explicit start delimiter — a literal <think> or <seed:think> is treated as a no-op rather than leaking into reasoning text.

I don't think this is the right direction. We'll detect there's already start/end token prefilled by checking the prompt tokens on initialize, so we're clear what the initial state is and there's no need to be as defensive as the Python impl.

I've also tested that by specifying default_in_reasoning = false both roundtrip tests can pass with official chat template.

[yzhan1]

@BugenZhao Thanks for the review. Updated based on your suggestions and cleaned up.

[BugenZhao]

Rest LGTM. Thanks!

[mergify]

Hi @yzhan1, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

[BugenZhao]

LGTM! Thanks

[yzhan1]

@BugenZhao Thanks! Seems like PR is stuck at the docs generation. Any ways we can bypass/fix this?