[ci][issue] Fix external contributor assignment

[CYFS3]

拉取/合并请求描述：(PR description)

[

为什么提交这份PR (why to submit this PR)

Issue 认领脚本使用仓库级 assignee 检查接口判断评论者能否被指派。该接口会将不属于仓库协作者的外部贡献者判定为不可指派，即使该用户已经评论了具体 Issue，并且 GitHub 的 Issue 级接口允许将其设为 Assignee。

这会导致外部贡献者评论 /claim 后，只能通过机器人评论和 in progress 标签记录认领状态，Issue 右侧的 Assignees 仍为空。

你的解决方案是什么 (what is your solution)

将可指派性检查改为使用包含 Issue 编号的 Issue 级 assignee 接口，并在认领流程中传入当前 Issue 编号。保留原有回退逻辑，确保 GitHub 确实不允许指派的账号仍可通过评论和标签记录认领状态。

新增单元测试验证 Issue 级 API 路径及 Issue 编号透传。

请提供验证的bsp和config (provide the config and bsp)

• BSP: 不涉及 BSP。

• .config: 不涉及配置变更。

• action: Issue Automation Tests；本地执行认领专项 22 项测试及 Issue 自动化完整 48 项测试，全部通过。

]

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

• 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
• 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

• 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
• 代码风格正确，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
• 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
• 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
• 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
• 代码是高质量的 Code in this PR is of high quality
• 已经使用clang-format 源码格式化工具确保格式符合RT-Thread代码规范 This PR has been formatted with clang-format and complies with RT-Thread code specification
• 如果是新增bsp, 已经添加ci检查到.github/ALL_BSP_COMPILE.json 详细请参考链接BSP自查

[github-actions]

👋 感谢您对 RT-Thread 的贡献！Thank you for your contribution to RT-Thread!

为确保代码符合 RT-Thread 的编码规范，请在你的仓库中执行以下步骤运行代码格式化工作流（如果格式化CI运行失败）。
To ensure your code complies with RT-Thread's coding style, please run the code formatting workflow by following the steps below (If the formatting of CI fails to run).

---

🛠 操作步骤 | Steps

1. 前往 Actions 页面 | Go to the Actions page
   点击进入工作流 → | Click to open workflow →

2. 点击 Run workflow | Click Run workflow

• 设置需排除的文件/目录（目录请以"/"结尾）
  Set files/directories to exclude (directories should end with "/")
• 将目标分支设置为 \ Set the target branch to：fix/issue-claim-assignee-check
• 设置PR number为 \ Set the PR number to：11666

3. 等待工作流完成 | Wait for the workflow to complete
   格式化后的代码将自动推送至你的分支。
   The formatted code will be automatically pushed to your branch.

完成后，提交将自动更新至 fix/issue-claim-assignee-check 分支，关联的 Pull Request 也会同步更新。
Once completed, commits will be pushed to the fix/issue-claim-assignee-check branch automatically, and the related Pull Request will be updated.

如有问题欢迎联系我们，再次感谢您的贡献！💐
If you have any questions, feel free to reach out. Thanks again for your contribution!

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• at32f415-start
• esp32-c3
• gd32105r-start
• hc32f334
• hpmicro-hpm5301evklite
• infineon-psoc6
• k230
• loongson-ls1cdev
• nordic-nrf51822
• nuvoton-m487
• nxp-lpc1114
• qemu-virt64-aarch64
• raspberry-pico-rp2040
• renesas-ra2l1
• simulator
• stm32f407-rt-spark
• stm32l475-atk-pandora-llvm
• wch-ch32v208w-r0
• x86
• xuantie-e901plus

[Copilot]

Pull request overview

This PR fixes the /claim issue-automation workflow so external contributors can be assigned when GitHub allows assignment at the issue level (even if they are not repository collaborators), by switching the assignability check to the issue-scoped assignee API.

本 PR 修复 Issue 自动化 /claim 流程：当 GitHub 在 Issue 级别允许将外部贡献者设为 Assignee（即使其不是仓库协作者）时，脚本也能正确完成指派；实现方式是将“可指派性”检查切换为 Issue 级 assignee 接口。

Changes:

• Update GitHubClient.can_assign() to call the issue-scoped endpoint /repos/{owner}/{repo}/issues/{issue_number}/assignees/{assignee} and thread issue_number through the claim flow.
• Add/extend unit tests to verify the new API path and that issue_number is passed into the assignability check.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tools/ci/issue_claim.py	Switches assignability check to issue-scoped API and passes issue_number through handle_claim().
tools/ci/test_issue_claim.py	Adds a unit test asserting the exact request path/expected status, and updates handler tests to assert issue_number is used.