[changelog] Fix Dependabot fallback branch validation (#191)

[coisa]

Related Issue

Closes #191

Motivation / Context

• Dependabot PRs could pass changelog validation after a rebase without actually having their own branch-specific changelog entry.
• The fallback action was deciding from the checked out PR merge ref instead of the real head branch, which let inherited Unreleased state look good enough.

Changes

• switch the Dependabot fallback action to fetch and evaluate the actual PR head branch before deciding whether an entry is already present
• make the action self-contained by fetching the base ref itself and emitting an explicit already-present, auto-created, or missing status
• fail the fallback action if it still cannot prove that the generated entry exists after creation
• add regression coverage for rebased branches that inherit unrelated Unreleased entries from main

Verification

• composer dev-tools
• Focused command(s):
  • ./vendor/bin/phpunit tests/Changelog/Checker/UnreleasedEntryCheckerTest.php tests/Changelog/DependabotChangelogEntryMessageResolverTest.php tests/Console/Command/ChangelogCheckCommandTest.php tests/Console/Command/ChangelogEntryCommandTest.php
  • bash -n .github/actions/changelog/create-dependabot-entry/run.sh
  • ruby -e 'require "yaml"; YAML.load_file(".github/actions/changelog/create-dependabot-entry/action.yml"); YAML.load_file(".github/workflows/changelog.yml"); puts "yaml ok"'
  • composer dev-tools changelog:check
  • git diff --check
• Manual verification:

Documentation / Generated Output

• README updated
• docs/ updated
• Generated or synchronized output reviewed

Changelog

• Added a notable CHANGELOG.md entry

Reviewer Notes

• The fix intentionally keeps the generic changelog checker as the source of truth for "meaningful branch-specific entry", but now runs that check from the real head branch instead of the PR merge checkout.

[github-actions]

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