feat(drivers/github): add optional accurate modified time

[jiwangyihao]

Description / 描述

• 为 GitHub Driver 新增默认关闭的 accurate_modified_time 配置项。
• 在 contents 小目录列表中使用 GitHub GraphQL 以 best-effort 方式回填真实 modified。
• 保持 legacy created 与 tree fallback 行为，并补齐 helper、行为测试与 op.List 缓存回归测试。

Motivation and Context / 背景

GitHub 的 contents / git/trees 列表接口本身不返回真实修改时间，当前 GitHub Driver 会把 Modified 固定为 zero-time，导致下游看到 1970。这个 PR 采用默认关闭、显式开启的增强方式，只在小目录上补齐准确 mtime，同时保持失败回退和请求成本边界。

How Has This Been Tested? / 测试

• gofmt -w drivers/github/meta.go drivers/github/types.go drivers/github/driver.go drivers/github/mtime.go drivers/github/mtime_test.go
• go test ./drivers/github
• 新增测试覆盖成功回填、disabled / missing token / 201 条目跳过、首批失败、第二批失败、rate-limit 止损、200 条目 4 个 batch、tree fallback 旧行为、op.List 缓存命中不重复 GraphQL。

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）。
• 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 #XXXX
  • OpenList-Docs #XXXX

[Copilot]

Pull request overview

This PR adds an opt-in enhancement to the GitHub driver to best-effort backfill accurate Modified timestamps for small directory listings via GitHub GraphQL, while preserving the legacy zero-time fallback behavior on failure and keeping created behavior unchanged.

Changes:

• Add accurate_modified_time config option (default false) to enable accurate mtime backfill for small listings.
• Implement GraphQL batched commit-history lookups and apply best-effort Modified timestamps during List.
• Add comprehensive behavior tests, including failure fallback, rate-limit stop, batching behavior, and op.List cache regression coverage.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
drivers/github/meta.go	Registers new accurate_modified_time driver config (default disabled) with help text.
drivers/github/driver.go	Hooks the mtime backfill into List for non-tree-fallback directory listings.
drivers/github/mtime.go	Implements GraphQL batching, response parsing, and best-effort application of accurate mtimes.
drivers/github/types.go	Uses a shared epoch sentinel (githubZeroTime) and sets Ctime to preserve legacy create-time behavior.
drivers/github/mtime_test.go	Adds tests for eligibility, batching, failure modes, rate-limit stopping, tree fallback behavior, and cache hit behavior.

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