feat(release): Include merged pull requests in release notes

[thomas-vilte]

Description

I implemented the functionality to include merged pull requests in the generated release notes. This change enriches the release notes by providing a clear list of PRs that contributed to the release, along with their titles, authors, and links.

Additionally, I refined the logic for fetching contributors and file statistics to ensure comparisons are made against the current branch tip (headRef) rather than a prospective, untagged version. This addresses potential issues when generating release notes for a version that hasn't been formally tagged yet.

I also updated the GitHub client's GetMergedPRsBetweenTags method to correctly identify merged pull requests by checking the MergedAt timestamp, as the Merged boolean field is not reliably populated by the GitHub API's list endpoint.

Fixes # (issue)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

I added a new unit test in internal/services/release_service_test.go to verify that contributors and file statistics are correctly compared against the current branch (master in the test case) when the release version is prospective.

Existing unit tests for GetMergedPRsBetweenTags in internal/vcs/github/client_test.go were updated to reflect the change in how merged PRs are identified via the MergedAt timestamp.

• Unit tests
• Integration tests
• Manual test: (describe below)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Test Plan & Evidence

Suggested Manual Verification

• API/Backend: Attach JSON response or logs as evidence of correct behavior
• Performance: Ensure no significant latency increase
• Unit Tests: Run go test ./... and ensure all tests pass
• No Regressions: Verify that related features still work as expected