Decouple EagerNode and Search via EagerEngine interface

[Copilot]

Removes hard dependency between ClusterNode and Search by introducing an EagerEngine interface, enabling alternative engine implementations.

Changes

• Added EagerEngine interface defining the engine contract:

  • input(data: string): string | null - returns formatted data or null on rejection
  • output(callback: (result: string) => void): number - executes search round
  • meta(): {input: string[], output: string[]} - exposes I/O patterns

• Renamed ClusterNode → EagerNode with constructor signature change:

  // Before
  constructor(addr: string, id: string = randomUUID(), limit_size: number = 1000, buffer_size: number = 10000)
  
  // After
  constructor(engine: EagerEngine, addr: string, id: string = randomUUID())

• Updated Search to implement EagerEngine (methods already matched interface)

• Replaced non-null assertions with null checks in all engine.input() call sites:

  // Before
  const formattedItem = this.engine.input(item)!;
  this.data.add(formattedItem);
  
  // After
  const formattedItem = this.engine.input(item);
  if (formattedItem !== null) {
      this.data.add(formattedItem);
  }

• Updated instantiation sites to inject Search instance:

  const engine = new Search();
  const node = new EagerNode(engine, listenAddr);

Usage

Engines can now be swapped at runtime:

class CustomEngine implements EagerEngine {
    input(data: string): string | null { /* ... */ }
    output(callback: (result: string) => void): number { /* ... */ }
    meta(): {input: string[], output: string[]} { /* ... */ }
}

const node = new EagerNode(new CustomEngine(), "0.0.0.0:8080");

Original prompt

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This section details on the original issue you should resolve

<issue_title>将 ClusterNode 和 Search 彻底解耦合</issue_title>
<issue_description>ClusterNode改名叫做 EagerNode， 他接收一个对象，实现了EagerEngine就行， 而Search实现了EagerEngine。

EagerEngine大概有这几个接口：

• input(string): string | null # 接收一个数据， 返回的如果是string就是接受了这个数据，并返回了格式化后的数据，如果是null就是不接受。所以现在有 this.engine.input(item)! 的地方不能直接用感叹号， 要判断非空了
• output((string) -> void): number # 进行一轮搜索，将结果调用callback
• meta(): {input: string[], output: string[]} # 提供当前状态下的输入输出模式

然后 EagerNode 的构造函数大概是 constructor(engine: EagerEngine, addr: string, id: string = randomUUID())</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes USTC-KnowledgeComputingLab/ddss#17

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Pull request overview

This PR successfully decouples the ClusterNode (now EagerNode) from the Search class by introducing an EagerEngine interface, enabling alternative engine implementations through dependency injection.

• Introduced EagerEngine interface defining the contract for engine implementations
• Renamed ClusterNode to EagerNode with constructor accepting an EagerEngine instance
• Replaced non-null assertions with proper null checks throughout the codebase

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