Custom pull-request branch prefix is incompatible with fetch-metadata

[ruzickap]

dependabot.yml content

version: 2
updates:
  - package-ecosystem: "github-actions"
    directory: "/"
    groups:
      dependencies:
        applies-to: version-updates
        update-types:
          - "minor"
          - "patch"
    pull-request-branch-name:
      prefix: "chore"                       ## <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
      word-separator: "-"
      branch-name-case: "lowercase"
    schedule:
      interval: "weekly"

What you expected to see, versus what you actually saw

Dependabot correctly created the update PR using the configured custom branch
prefix:

chore/github-actions/dependencies-fca3083807

I expected dependabot/fetch-metadata to extract the metadata from this
verified Dependabot PR.

Instead, it failed with:

Parsing Dependabot metadata
Error: PR does not contain metadata, nothing to do.

fetch-metadata appears to require the branch name to start with
dependabot.

This conflicts with the supported pull-request-branch-name.prefix
configuration. With the custom chore prefix, the valid commit metadata is
ignored.

• Expected behavior: metadata extraction should support branch prefixes generated
  through the documented pull-request-branch-name configuration.

• Actual behavior: fetch-metadata fails because the generated branch does not
  start with dependabot.

Smallest manifest that reproduces the issue

1. Configure a GitHub Actions update with:

pull-request-branch-name:
  prefix: "chore"

2. Let Dependabot open an update PR.
3. Run:

- name: Fetch Dependabot metadata
  uses: dependabot/fetch-metadata@v3

The generated branch starts with chore/, and metadata extraction fails with
PR does not contain metadata, nothing to do.