False warning for Github Action checkout step: "Context access might be invalid"

**Describe the bug**
False warning for Github checkout step: "Context access might be invalid"

**To Reproduce**
Steps to reproduce the behavior:
1. With this workflow '...'
- name: checkout
if: github.event_name == 'pull_request' || github.event_name == 'pull_request_target'
uses: actions/checkout@v4
with:
ref: ${{ github.event.pull_request.head.sha }}
I get this warning under problems in VS Code:
"Context access might be invalid: pull_request"

**Expected behavior**
No problem warning in VS Code because I am checking if github event is pull_request or pull_request_target

**Extension Version**
`v0.27.1`

Start by reproducing the warning with the workflow snippet in the issue and inspect the reported VS Code problem for the github.event.pull_request context. Trace the extension's context validation behavior for pull_request and pull_request_target; done means the valid guarded access no longer produces a warning.

由索引模型根据 Issue 内容生成。