feat: Add PR message formatting for dependency-name groups

[markhallen]

What are you trying to accomplish?

When group-by: dependency-name is configured, dynamic subgroups like "monorepo-deps/dummy-pkg-a" are created. Today the raw subgroup name leaks into PR titles and bodies, producing confusing messages like "Bump the monorepo-deps/dummy-pkg-a group across 3 directories".

This PR adds dedicated title and body formatting for these dependency-name groups so that users see clean, dependency-focused messages instead.

PR title examples:

Scenario	Title
Multiple directories	Bump dummy-pkg-a across 3 directories
Single directory	Bump dummy-pkg-a in /frontend
No directory info	Bump dummy-pkg-a

PR body lists each affected directory with its version change (e.g. - /frontend: 1.4.0 → 1.5.0).

Anything you want to highlight for special attention from reviewers?

The change is split into two layers:

1. Metadata plumbing (GroupDependencySelector#deduplicate_by_name_only): When deduplicating by name, the surviving dependency now gets metadata[:updated_directories] populated with all directories where it was updated. This preserves directory info that was previously lost.

2. Message formatting (MessageBuilder): New dependency_name_group_pr_name and dependency_name_group_intro methods dispatch from the existing group_pr_name and version_commit_message_intro methods when dependency_group.group_by_dependency_name? is true. All existing code paths are unchanged.

How will you know you've accomplished your goal?

• All existing tests continue to pass (44/44 selector tests, 128/128 message builder tests)
• New tests verify:
  • updated_directories metadata is populated with all directories when group_by_dependency_name? is true
  • updated_directories is not set when group_by_dependency_name? is false (no behavior change)
  • PR title formats correctly for multi-directory, single-directory, and no-directory scenarios
  • PR body formats correctly with per-directory version listings

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

[Copilot]

Pull request overview

This PR adds dedicated formatting for dependency-name groups (created when group-by: dependency-name is configured) to produce clean, dependency-focused PR titles and bodies instead of exposing the raw subgroup names.

Changes:

• Enhanced GroupDependencySelector to preserve directory information in metadata when deduplicating by dependency name
• Added dependency_name_group_pr_name and dependency_name_group_intro methods to MessageBuilder for dependency-name group formatting
• Added comprehensive test coverage for both single and multi-directory scenarios

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
updater/lib/dependabot/updater/group_dependency_selector.rb	Added logic to populate updated_directories metadata when deduplicating by name, preserving directory context
updater/spec/dependabot/updater/group_dependency_selector_spec.rb	Added tests verifying metadata population for dependency-name groups and absence of metadata for regular groups
common/lib/dependabot/pull_request_creator/message_builder.rb	Added new formatting methods for dependency-name group PR titles and bodies with directory-aware messaging
common/spec/dependabot/pull_request_creator/message_builder_spec.rb	Added tests for PR title and body formatting across multiple directory scenarios