feat(types): Add GitLab Resource Event serde types with deserialization tests

Adds six new types for deserializing responses from GitLab's three
Resource Events API endpoints (state, label, milestone):

- GitLabStateEvent: State transitions with optional user, source_commit,
  and source_merge_request reference
- GitLabLabelEvent: Label add/remove events with nested GitLabLabelRef
- GitLabMilestoneEvent: Milestone assignment changes with nested
  GitLabMilestoneRef
- GitLabMergeRequestRef: Lightweight MR reference (iid, title, web_url)
- GitLabLabelRef: Label metadata (id, name, color, description)
- GitLabMilestoneRef: Milestone metadata (id, iid, title)

All types derive Deserialize + Serialize and use Option<T> for nullable
fields (user, source_commit, color, description) to match GitLab's API
contract where these fields may be null.

Includes 8 new test cases covering:
- State events with/without user, with/without source_merge_request
- Label events for add and remove actions, including null color handling
- Milestone event deserialization
- Standalone ref type deserialization (MR, label, milestone)

Uses r##"..."## raw string delimiters where JSON contains hex color
codes (#FF0000) that would conflict with r#"..."# delimiters.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/gitlab/mod.rs

@@ -10,6 +10,7 @@ pub use transformers::{
     transform_discussion, transform_issue, transform_notes,
 };
 pub use types::{
-    GitLabAuthor, GitLabDiscussion, GitLabIssue, GitLabNote, GitLabNotePosition, GitLabProject,
-    GitLabUser, GitLabVersion,
+    GitLabAuthor, GitLabDiscussion, GitLabIssue, GitLabLabelEvent, GitLabLabelRef,
+    GitLabMergeRequestRef, GitLabMilestoneEvent, GitLabMilestoneRef, GitLabNote,
+    GitLabNotePosition, GitLabProject, GitLabStateEvent, GitLabUser, GitLabVersion,
 };