Read-only editors for action.yml shows problems as if it were a workflow

**Describe the bug**

Some diffs (but not all!) show a load of false positive Problems for `action.yml` treating it as if it were a workflow YAML file.

Possibly this only affects read-only diffs.

**To Reproduce**
Steps to reproduce the behavior:
1. Open a read-only diff of an `action.yml` file

**Expected behavior**
`action.yml` should be correctly identified as an action not a workflow, in diff views

**Screenshots**

Top: an unaffected diff; bottom: an affected read-only diff

**Extension Version**
`0.32.3`

**Additional context**
(Originally opened in https://github.com/microsoft/vscode-pull-request-github/issues/8873 but it affects diffs unrelated to PRs)

action.yml ファイルの読み取り専用 diff を開き、誤検知された Problems を確認して問題を再現します。まず、diff ビューで拡張機能が action メタデータと workflow YAML をどのように区別しているかを追跡し、次にそのファイルが action として識別され、workflow 固有の Problems が表示されないことを確認します。

索引モデルが issue の本文から書いたものです。