fix(gitlab): resolve commit author usernames from API

[leno23]

Summary

• GitLab's commits API returns author_name (display name) rather than a username, so changelogs showed @Nathan Belsterling instead of @nbelste1.
• Resolve usernames after fetching commits by:
  1. Mapping merge commit SHAs to merge request author usernames
  2. Looking up remaining authors via project members (/members/all?query=email) and user search fallback
• Store the resolved value in GitLabCommit.resolved_username and use it for commit.gitlab.username template variables.

Fixes #1492

Test plan

• cargo test -p git-cliff-core --features gitlab gitlab
• Added unit tests for resolved username, MR mapping, and members URL encoding
• GitLab integration fixture (test-gitlab-integration) in CI

[codecov-commenter]

Codecov Report

❌ Patch coverage is 34.16667% with 79 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.67%. Comparing base (7e35ff5) to head (623e0db).

Files with missing lines	Patch %	Lines
git-cliff-core/src/remote/gitlab.rs	26.39%	53 Missing ⚠️
git-cliff-core/src/changelog.rs	45.84%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1540      +/-   ##
==========================================
- Coverage   48.63%   47.67%   -0.96%     
==========================================
  Files          26       26              
  Lines        2287     2373      +86     
==========================================
+ Hits         1112     1131      +19     
- Misses       1175     1242      +67     

Flag	Coverage Δ
unit-tests	47.67% <34.17%> (-0.96%)	⬇️

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