PR overview remains stuck checking mergeability after an UNKNOWN response

定位调用 checkMergeability() 和 setMergeability() 的轮询代码，然后检查 mergeability 响应是如何传递到 merge-status 视图中的。更新处理逻辑，使计算出的 mergeability 替换不确定的检查状态，并验证 GitHub 返回已确定的状态后 pull request 概览会更新。

• Extension version: 0.166.0 and 0.167.2026091104 (pre-release)
• VSCode Version: 1.137.0
• OS: macOS 26.6.2 (25G83), arm64
• Repository Clone Configuration: Single repository
• GitHub Product: GitHub.com

Steps to Reproduce:

1. Open a repository containing an open pull request.
2. Open the pull request description from the GitHub Pull Requests view while GitHub reports its mergeability as UNKNOWN, such as immediately after creating or updating the pull request.
3. Scroll to the merge-status section and wait for GitHub to finish calculating mergeability.

Expected:

The extension polls again and replaces “Checking if this branch can be merged...” with the calculated mergeability status.

Actual:

“Checking if this branch can be merged...” remains indefinitely, even after GitHub.com and the GitHub API report the pull request as mergeable.

The polling code receives an object shaped like { mergeability, conflicts }, but passes the entire object to setMergeability():

const newMergeability = await checkMergeability();
setMergeability(newMergeability);

It should use:

setMergeability(newMergeability.mergeability);

Related: #4118 reported the same symptom but was closed before this response-shape regression was introduced.