[PD Disaggregation] decode does not cache requests in pd

[juncaipeng]

Motivation

decode 不支持缓存申请资源的 cache

Modifications

删除FD_ENABLE_CACHE_TASK环境变量
修改 PD 交互逻辑

Usage or Command

无

Accuracy Tests

单侧覆盖

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!

[Copilot]

Pull request overview

此 PR 移除了 FD_ENABLE_CACHE_TASK 环境变量及其相关逻辑，简化了 PD（Prefill-Decode）分离架构中 decode 节点的资源申请缓存机制。主要变更包括删除环境变量定义、移除相关的条件判断逻辑，并统一了资源分配失败时的处理流程。

主要变更：

• 移除 FD_ENABLE_CACHE_TASK 环境变量及其在代码中的所有引用
• 简化 decode 节点的资源分配请求处理逻辑，统一资源不足时的错误处理
• 改进错误消息的清晰度

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
fastdeploy/envs.py	删除 FD_ENABLE_CACHE_TASK 环境变量定义
fastdeploy/splitwise/splitwise_connector.py	移除 enable_decode_cache_task 的初始化和条件判断逻辑，改进超时错误消息，优化 check_decode_allocated 和 _process_message 方法
fastdeploy/engine/common_engine.py	移除 enable_decode_cache_task 的初始化，重构 _process_allocate_resource_requests 函数以简化资源分配逻辑，改进变量命名

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (develop@80efe98). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/engine/common_engine.py	0.00%	15 Missing ⚠️
fastdeploy/splitwise/splitwise_connector.py	0.00%	3 Missing ⚠️

Additional details and impacted files

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