[Feature] Support ThinkingBudget Logits processor to control thinking content length

[jackyYang6]

Motivation

引入 ThinkingBudget logits processor 用于限制 段长度，同时补充中英文使用文档与单测。

Modifications

• 新增 ThinkingBudgetLogitsProcessor 并注册为内置 logits processor，用于控制思考长度。
• DataProcessor 阶段进行 <think> prompt 状态预计算，避免首步 GPU 扫描 prompt。
• 打通 Engine/SamplingParams 的参数透传链路。
• 新增 thinking budget 行为单测。
• 新增中英文功能文档并更新 mkdocs 导航。

Usage or Command

启动服务：

python -m fastdeploy.entrypoints.openai.api_server \
  --model Qwen/Qwen3-0.6B \
  --port 8180 \
  --metrics-port 8181 \
  --engine-worker-queue-port 8182 \
  --max-model-len 32768 \
  --max-num-seqs 32 \
  --logits-processors ThinkingBudgetLogitsProcessor

请求示例：

curl -X POST "http://0.0.0.0:8180/v1/chat/completions" \
  -H "Content-Type: application/json" \
  -d '{
    "messages": [{"role": "user", "content": "你好！"}],
    "max_completion_tokens": 30,
    "logits_processors_args": {
      "thinking_budget": 20,
      "think_stop_sentence": "思考已达上限，开始回复"
    }
  }'

Accuracy Tests

不涉及 kernel 或模型 forward 逻辑变更，暂无精度回归需求。

Checklist

• Add at least a tag in the PR title.
  • Tag list: [[FDConfig],[APIServer],[Engine], [Scheduler], [PD Disaggregation], [Executor], [Graph Optimization], [Speculative Decoding], [RL], [Models], [Quantization], [Loader], [OP], [KVCache], [DataProcessor], [BugFix], [Docs], [CI], [Optimization], [Feature], [Benchmark], [Others], [XPU], [HPU], [GCU], [DCU], [Iluvatar], [Metax]]
  • You can add new tags based on the PR content, but the semantics must be clear.
• Format your code, run pre-commit before commit.
• Add unit tests. Please write the reason in this PR if no unit tests.
• Provide accuracy results.
• If the current PR is submitting to the release branch, make sure the PR has been submitted to the develop branch, then cherry-pick it to the release branch with the [Cherry-Pick] PR tag.

[paddle-bot]

Thanks for your contribution!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 63.65591% with 169 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (develop@72fe94c). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/input/v1/text_processor.py	22.82%	67 Missing and 4 partials ⚠️
fastdeploy/input/text_processor.py	50.00%	35 Missing and 13 partials ⚠️
...model_executor/logits_processor/thinking_budget.py	84.61%	14 Missing and 22 partials ⚠️
fastdeploy/engine/common_engine.py	65.00%	2 Missing and 5 partials ⚠️
fastdeploy/engine/engine.py	65.00%	2 Missing and 5 partials ⚠️

Additional details and impacted files

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==========================================
  Files              ?      392           
  Lines              ?    52628           
  Branches           ?     8240           
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  Hits               ?    35602           
  Misses             ?    14382           
  Partials           ?     2644           

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