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)

Reproduce the issue by opening a read-only diff for an action.yml file and observe the false-positive Problems. Start by tracing how the extension distinguishes action metadata from workflow YAML in diff views, then verify that the file is identified as an action and no workflow-specific Problems appear.

Written by the indexing model from the issue text.