perf(index): optimize search index slice allocation

[MEKXH]

perf(search): optimize search index slice allocation

Description / 描述

优化搜索索引构建过程中的切片内存分配策略：

1. build.go: 在 batchIndexMessages 函数中使用预分配切片替代 append 动态增长
2. search.go: 在 BatchIndex 函数中同样采用预分配策略，移除 utils.MustSliceConvert 中间转换

核心变更

- var searchNodes []model.SearchNode
- for i := range objs {
-     searchNodes = append(searchNodes, model.SearchNode{...})
- }
+ searchNodes := make([]model.SearchNode, len(objs))
+ for i := range objs {
+     searchNodes[i] = model.SearchNode{...}
+ }

Motivation and Context / 背景

搜索索引批量构建是一个高频操作，尤其在初始化索引或批量更新时。原实现存在以下性能问题：

1. append 动态扩容: 导致多次内存分配和数据拷贝
2. utils.MustSliceConvert 中间切片: 产生额外内存分配开销

通过预分配确切大小的切片，可显著减少 GC 压力并提升批量索引性能。

Relates to performance optimization

How Has This Been Tested? / 测试

添加了 Benchmark 测试对比优化前后性能：

go test -bench=BenchmarkSearchAllocation -benchmem ./internal/search/

测试结果

数据规模	旧逻辑 (ns/op)	新逻辑 (ns/op)	内存分配次数	提升
100	~5841	~486	12 → 1	~12x
1000	~43277	~15637	12 → 1	~2.8x
10000	~470000	~177000	12 → 1	~2.6x

Checklist / 检查清单

• I have read the CONTRIBUTING document.
  我已阅读 CONTRIBUTING 文档。
• I have formatted my code with go fmt or prettier.
  我已使用 go fmt 或 prettier 格式化提交的代码。
• I have added appropriate labels to this PR (or mentioned needed labels in the description if lacking permissions).
  我已为此 PR 添加了适当的标签（如无权限或需要的标签不存在，请在描述中说明，管理员将后续处理）。
  建议标签: performance, search
• I have requested review from relevant code authors using the "Request review" feature when applicable.
  我已在适当情况下使用"Request review"功能请求相关代码作者进行审查。
• I have updated the repository accordingly (If it's needed).
  我已相应更新了相关仓库（若适用）。
  • OpenList-Frontend - 无需更新
  • OpenList-Docs - 无需更新

[Copilot]

Pull request overview

This PR optimizes search index batch-building by reducing slice reallocations and avoiding intermediate slice conversions, with an added benchmark to quantify allocation improvements.

Changes:

• Preallocate []model.SearchNode in BatchIndex to avoid append growth.
• Replace utils.MustSliceConvert + BatchIndex in index building with a direct preallocated conversion path for MQ messages.
• Add benchmarks to compare allocation behavior before/after the optimization.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
internal/search/search.go	Preallocates searchNodes in BatchIndex and assigns by index to reduce allocations.
internal/search/build.go	Introduces batchIndexMessages to build searchNodes directly from MQ messages and preallocates toAddObjs capacity.
internal/search/benchmark_test.go	Adds benchmarks simulating old vs new slice allocation strategies.

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