[GFX-724] fix(runtime): PathHelper.Combine 绝对路径段正确追加分隔符

[AlianBlank]

Summary

• 删除 Runtime/Helper/PathHelper.cs 中 Combine 非末段拼接的 StartsWithFast(separator) 跳过分支：以 / 开头且不以分隔符结尾的绝对路径段不再与后段粘连（/Users/blank/cache + img.png 旧产出 /Users/blank/cacheimg.png）
• 纯 / 段、尾 /、尾 \ 段行为不变（仍由 EndsWith 判定覆盖）；对外 API 签名不变
• Tests/UtilityPathTests.cs 新增 #region PathHelper.Combine 七个回归用例（对照 GameFrameX.Godot 2513f60 用例语义）

Test plan

• 独立差分验证（dotnet 10 + 真实源码 + UnityEngine shim）：修复前 7 用例 2 败（两个绝对路径用例，复现 issue 现象）→ 修复后 7/7 通过；其余 5 类输入修复前后输出逐字节一致
• 编译烟测：完整修改版 UtilityPathTests.cs + 修复后 PathHelper + Utility.Path 门面对 Unity nunit.framework.dll 编译 0 错误
• Unity Editor 导入包无编译报错；Test Runner 执行新增 7 用例（本环境无法启动 Editor，人工项）
• imagecache / startup 等消费方绝对路径场景落盘位置正确（附带纠正包内 BuildProductHelper.cs:270 传 projectDir.Parent.FullName 绝对路径段的同款粘连）

Linear: GFX-724

Summary by CodeRabbit

• Bug Fixes

  • 修复路径拼接时分隔符判断错误的问题。
  • 现在会为缺少末尾分隔符的路径段正确添加分隔符，并避免重复添加。

• Tests

  • 新增多项路径拼接测试，覆盖相对路径、绝对路径及不同斜杠组合等场景。

[linear-code]

GFX-724

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 7f029070-c067-493f-92f4-58b47e66dd1a

📥 Commits

Reviewing files that changed from the base of the PR and between 2fae759 and fd57db5.

📒 Files selected for processing (2)

• Runtime/Helper/PathHelper.cs
• Tests/UtilityPathTests.cs

💤 Files with no reviewable changes (1)

• Runtime/Helper/PathHelper.cs

Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review.

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📝 Walkthrough

Walkthrough

PathHelper.Combine 现在检查路径段末尾的分隔符，并为需要拼接的路径段追加分隔符。UtilityPathTests 新增 7 个测试，覆盖多种路径组合场景。

Changes

路径拼接行为

Layer / File(s)	Summary
组合路径行为与测试 Runtime/Helper/PathHelper.cs, Tests/UtilityPathTests.cs	PathHelper.Combine 根据路径段末尾判断 / 或 \。测试覆盖单段路径、相对路径、绝对路径、尾随分隔符及多段路径。

Estimated code review effort: 2 (简单) | ~10 分钟

Merge Risk: ⚪ Minimal · up to fd57d

This change fixes absolute-path joining so subsequent path segments are separated correctly while preserving existing trailing-separator behavior; no actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 8 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	标题准确描述了主要变更：修复 PathHelper.Combine 对绝对路径段追加分隔符的问题。标题简洁、具体，并包含关联任务编号。
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/gfx-724

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